Model type expressions as regions (#4698)

This is a precondition for enabling the new pattern-matching subsystem
to support binding patterns that have `if` expressions in the type
position.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_if_expr.cpp

@@ -25,7 +25,7 @@ auto HandleParseNode(Context& context, Parse::IfExprIfId node_id) -> bool {
   // Start emitting the `then` block.
   context.inst_block_stack().Pop();
   context.inst_block_stack().Push(then_block_id);
-  context.AddCurrentCodeBlockToFunction(node_id);
+  context.AddToRegion(then_block_id, node_id);
 
   context.node_stack().Push(if_node, else_block_id);
   return true;
@@ -56,13 +56,18 @@ auto HandleParseNode(Context& context, Parse::IfExprThenId node_id) -> bool {
 
   // Start emitting the `else` block.
   context.inst_block_stack().Push(else_block_id);
-  context.AddCurrentCodeBlockToFunction(node_id);
+  context.AddToRegion(else_block_id, node_id);
 
   context.node_stack().Push(node_id, then_value_id);
   return true;
 }
 
 auto HandleParseNode(Context& context, Parse::IfExprElseId node_id) -> bool {
+  if (context.return_scope_stack().empty()) {
+    context.TODO(node_id,
+                 "Control flow expressions are currently only supported inside "
+                 "functions.");
+  }
   // Alias node_id for if/then/else consistency.
   auto& else_node = node_id;
 
@@ -84,7 +89,6 @@ auto HandleParseNode(Context& context, Parse::IfExprElseId node_id) -> bool {
       if_node, {else_value_id, then_value_id});
   context.SetBlockArgResultBeforeConstantUse(chosen_value_id, cond_value_id,
                                              then_value_id, else_value_id);
-  context.AddCurrentCodeBlockToFunction(node_id);
 
   // Push the result value.
   context.node_stack().Push(else_node, chosen_value_id);