Semantic analysis for if expressions (#2893)

Add semantic analysis and semantics IR building for `if` expressions, and add the first parts of control flow handling to semantics IR. After discussion with @chandlerc, use [block arguments](https://en.wikipedia.org/wiki/Static_single-assignment_form#Block_arguments) to convey values from the two arms of the `if` to the result. For now, only a single block argument is supported, but we should revisit this as we explore more of the requirements of the Semantics IR form.

Functions can now contain multiple code blocks, so grab the entry block up-front instead of assuming the entry block will be at the top of the block stack when we reach the end of function emission.

Add trivial support for `bool` type literal, because without it we can't write testcases.

toolchain/parser/testdata/if_expression/basic.carbon

@@ -8,7 +8,7 @@
 // CHECK:STDOUT:     {kind: 'DeclaredName', text: 'F'},
 // CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
 // CHECK:STDOUT:         {kind: 'DeclaredName', text: 'b'},
-// CHECK:STDOUT:         {kind: 'NameReference', text: 'bool'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'bool'},
 // CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
 // CHECK:STDOUT:       {kind: 'ParameterListComma', text: ','},
 // CHECK:STDOUT:         {kind: 'DeclaredName', text: 'x'},
@@ -19,16 +19,16 @@
 // CHECK:STDOUT:         {kind: 'Literal', text: 'i32'},
 // CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
 // CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 13},
-// CHECK:STDOUT:       {kind: 'NameReference', text: 'bool'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'bool'},
 // CHECK:STDOUT:     {kind: 'ReturnType', text: '->', subtree_size: 2},
 // CHECK:STDOUT:   {kind: 'FunctionDefinitionStart', text: '{', subtree_size: 18},
 // CHECK:STDOUT:     {kind: 'ReturnStatementStart', text: 'return'},
-// CHECK:STDOUT:       {kind: 'NameReference', text: 'b'},
-// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then'},
-// CHECK:STDOUT:       {kind: 'NameReference', text: 'x'},
-// CHECK:STDOUT:       {kind: 'IfExpressionElse', text: 'else'},
+// CHECK:STDOUT:         {kind: 'NameReference', text: 'b'},
+// CHECK:STDOUT:       {kind: 'IfExpressionIf', text: 'if', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'NameReference', text: 'x'},
+// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then', subtree_size: 2},
 // CHECK:STDOUT:       {kind: 'NameReference', text: 'y'},
-// CHECK:STDOUT:     {kind: 'IfExpression', text: 'if', subtree_size: 6},
+// CHECK:STDOUT:     {kind: 'IfExpressionElse', text: 'else', subtree_size: 6},
 // CHECK:STDOUT:   {kind: 'ReturnStatement', text: ';', subtree_size: 8},
 // CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 27},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},

toolchain/parser/testdata/if_expression/fail_condition_missing.carbon

@@ -14,14 +14,13 @@
 // CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
 // CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
 // CHECK:STDOUT:     {kind: 'VariableInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'InvalidParse', text: ';', has_error: yes},
+// CHECK:STDOUT:       {kind: 'IfExpressionIf', text: 'if', has_error: yes, subtree_size: 2},
 // CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
 // CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:     {kind: 'IfExpression', text: 'if', has_error: yes, subtree_size: 6},
-// CHECK:STDOUT:   {kind: 'VariableDeclaration', text: ';', subtree_size: 12},
-// CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 18},
+// CHECK:STDOUT:     {kind: 'IfExpressionElse', text: 'if', has_error: yes, subtree_size: 5},
+// CHECK:STDOUT:   {kind: 'VariableDeclaration', text: ';', subtree_size: 11},
+// CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 17},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 

toolchain/parser/testdata/if_expression/fail_else_expr_missing.carbon

@@ -14,12 +14,12 @@
 // CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
 // CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
 // CHECK:STDOUT:     {kind: 'VariableInitializer', text: '='},
-// CHECK:STDOUT:       {kind: 'NameReference', text: 'true'},
-// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then'},
-// CHECK:STDOUT:       {kind: 'Literal', text: '1'},
-// CHECK:STDOUT:       {kind: 'IfExpressionElse', text: 'else'},
+// CHECK:STDOUT:         {kind: 'NameReference', text: 'true'},
+// CHECK:STDOUT:       {kind: 'IfExpressionIf', text: 'if', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'Literal', text: '1'},
+// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then', subtree_size: 2},
 // CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:     {kind: 'IfExpression', text: 'if', has_error: yes, subtree_size: 6},
+// CHECK:STDOUT:     {kind: 'IfExpressionElse', text: 'else', has_error: yes, subtree_size: 6},
 // CHECK:STDOUT:   {kind: 'VariableDeclaration', text: ';', subtree_size: 12},
 // CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 18},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},

toolchain/parser/testdata/if_expression/fail_else_missing.carbon

@@ -14,12 +14,12 @@
 // CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
 // CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
 // CHECK:STDOUT:     {kind: 'VariableInitializer', text: '='},
-// CHECK:STDOUT:       {kind: 'NameReference', text: 'true'},
-// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then'},
-// CHECK:STDOUT:       {kind: 'Literal', text: '1'},
+// CHECK:STDOUT:         {kind: 'NameReference', text: 'true'},
+// CHECK:STDOUT:       {kind: 'IfExpressionIf', text: 'if', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'Literal', text: '1'},
+// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then', subtree_size: 2},
 // CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:     {kind: 'IfExpression', text: 'if', has_error: yes, subtree_size: 6},
+// CHECK:STDOUT:     {kind: 'IfExpressionElse', text: 'if', has_error: yes, subtree_size: 6},
 // CHECK:STDOUT:   {kind: 'VariableDeclaration', text: ';', subtree_size: 12},
 // CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 18},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},

toolchain/parser/testdata/if_expression/fail_then_expr_missing.carbon

@@ -14,12 +14,12 @@
 // CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
 // CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
 // CHECK:STDOUT:     {kind: 'VariableInitializer', text: '='},
-// CHECK:STDOUT:       {kind: 'NameReference', text: 'true'},
-// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then'},
+// CHECK:STDOUT:         {kind: 'NameReference', text: 'true'},
+// CHECK:STDOUT:       {kind: 'IfExpressionIf', text: 'if', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'InvalidParse', text: ';', has_error: yes},
+// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then', has_error: yes, subtree_size: 2},
 // CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:     {kind: 'IfExpression', text: 'if', has_error: yes, subtree_size: 6},
+// CHECK:STDOUT:     {kind: 'IfExpressionElse', text: 'if', has_error: yes, subtree_size: 6},
 // CHECK:STDOUT:   {kind: 'VariableDeclaration', text: ';', subtree_size: 12},
 // CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 18},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},

toolchain/parser/testdata/if_expression/fail_then_missing.carbon

@@ -14,14 +14,13 @@
 // CHECK:STDOUT:       {kind: 'Literal', text: 'i32'},
 // CHECK:STDOUT:     {kind: 'PatternBinding', text: ':', subtree_size: 3},
 // CHECK:STDOUT:     {kind: 'VariableInitializer', text: '='},
-// CHECK:STDOUT:       {kind: 'NameReference', text: 'true'},
+// CHECK:STDOUT:         {kind: 'NameReference', text: 'true'},
+// CHECK:STDOUT:       {kind: 'IfExpressionIf', text: 'if', subtree_size: 2},
 // CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
 // CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:       {kind: 'InvalidParse', text: ';', has_error: yes},
-// CHECK:STDOUT:     {kind: 'IfExpression', text: 'if', has_error: yes, subtree_size: 6},
-// CHECK:STDOUT:   {kind: 'VariableDeclaration', text: ';', subtree_size: 12},
-// CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 18},
+// CHECK:STDOUT:     {kind: 'IfExpressionElse', text: 'if', has_error: yes, subtree_size: 5},
+// CHECK:STDOUT:   {kind: 'VariableDeclaration', text: ';', subtree_size: 11},
+// CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 17},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 

toolchain/parser/testdata/if_expression/precedence.carbon

@@ -8,25 +8,25 @@
 // CHECK:STDOUT:     {kind: 'DeclaredName', text: 'F'},
 // CHECK:STDOUT:       {kind: 'ParameterListStart', text: '('},
 // CHECK:STDOUT:         {kind: 'DeclaredName', text: 'b'},
-// CHECK:STDOUT:         {kind: 'NameReference', text: 'bool'},
+// CHECK:STDOUT:         {kind: 'Literal', text: 'bool'},
 // CHECK:STDOUT:       {kind: 'PatternBinding', text: ':', subtree_size: 3},
 // CHECK:STDOUT:     {kind: 'ParameterList', text: ')', subtree_size: 5},
-// CHECK:STDOUT:       {kind: 'NameReference', text: 'bool'},
+// CHECK:STDOUT:       {kind: 'Literal', text: 'bool'},
 // CHECK:STDOUT:     {kind: 'ReturnType', text: '->', subtree_size: 2},
 // CHECK:STDOUT:   {kind: 'FunctionDefinitionStart', text: '{', subtree_size: 10},
 // CHECK:STDOUT:     {kind: 'ReturnStatementStart', text: 'return'},
-// CHECK:STDOUT:         {kind: 'NameReference', text: 'b'},
-// CHECK:STDOUT:         {kind: 'NameReference', text: 'b'},
-// CHECK:STDOUT:       {kind: 'InfixOperator', text: 'and', subtree_size: 3},
-// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then'},
-// CHECK:STDOUT:         {kind: 'NameReference', text: 'b'},
-// CHECK:STDOUT:         {kind: 'NameReference', text: 'b'},
-// CHECK:STDOUT:       {kind: 'InfixOperator', text: 'and', subtree_size: 3},
-// CHECK:STDOUT:       {kind: 'IfExpressionElse', text: 'else'},
+// CHECK:STDOUT:           {kind: 'NameReference', text: 'b'},
+// CHECK:STDOUT:           {kind: 'NameReference', text: 'b'},
+// CHECK:STDOUT:         {kind: 'InfixOperator', text: 'and', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'IfExpressionIf', text: 'if', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'NameReference', text: 'b'},
+// CHECK:STDOUT:           {kind: 'NameReference', text: 'b'},
+// CHECK:STDOUT:         {kind: 'InfixOperator', text: 'and', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'IfExpressionThen', text: 'then', subtree_size: 4},
 // CHECK:STDOUT:         {kind: 'NameReference', text: 'b'},
 // CHECK:STDOUT:         {kind: 'NameReference', text: 'b'},
 // CHECK:STDOUT:       {kind: 'InfixOperator', text: 'or', subtree_size: 3},
-// CHECK:STDOUT:     {kind: 'IfExpression', text: 'if', subtree_size: 12},
+// CHECK:STDOUT:     {kind: 'IfExpressionElse', text: 'else', subtree_size: 12},
 // CHECK:STDOUT:   {kind: 'ReturnStatement', text: ';', subtree_size: 14},
 // CHECK:STDOUT: {kind: 'FunctionDefinition', text: '}', subtree_size: 25},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},