Rename files to use abbreviations (#3389)

Reflects these recent PRs:
* #3375 
* #3382

toolchain/check/handle_if_expr.cpp

@@ -0,0 +1,72 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/context.h"
+#include "toolchain/check/convert.h"
+
+namespace Carbon::Check {
+
+auto HandleIfExprIf(Context& context, Parse::Node parse_node) -> bool {
+  // Alias parse_node for if/then/else consistency.
+  auto& if_node = parse_node;
+
+  auto cond_value_id = context.node_stack().PopExpr();
+
+  // Convert the condition to `bool`, and branch on it.
+  cond_value_id = ConvertToBoolValue(context, if_node, cond_value_id);
+  auto then_block_id =
+      context.AddDominatedBlockAndBranchIf(if_node, cond_value_id);
+  auto else_block_id = context.AddDominatedBlockAndBranch(if_node);
+
+  // Start emitting the `then` block.
+  context.inst_block_stack().Pop();
+  context.inst_block_stack().Push(then_block_id);
+  context.AddCurrentCodeBlockToFunction(parse_node);
+
+  context.node_stack().Push(if_node, else_block_id);
+  return true;
+}
+
+auto HandleIfExprThen(Context& context, Parse::Node parse_node) -> bool {
+  auto then_value_id = context.node_stack().PopExpr();
+  auto else_block_id = context.node_stack().Peek<Parse::NodeKind::IfExprIf>();
+
+  // Convert the first operand to a value.
+  then_value_id = ConvertToValueExpr(context, then_value_id);
+
+  // Start emitting the `else` block.
+  context.inst_block_stack().Push(else_block_id);
+  context.AddCurrentCodeBlockToFunction(parse_node);
+
+  context.node_stack().Push(parse_node, then_value_id);
+  return true;
+}
+
+auto HandleIfExprElse(Context& context, Parse::Node parse_node) -> bool {
+  // Alias parse_node for if/then/else consistency.
+  auto& else_node = parse_node;
+
+  auto else_value_id = context.node_stack().PopExpr();
+  auto then_value_id = context.node_stack().Pop<Parse::NodeKind::IfExprThen>();
+  auto [if_node, _] =
+      context.node_stack().PopWithParseNode<Parse::NodeKind::IfExprIf>();
+
+  // Convert the `else` value to the `then` value's type, and finish the `else`
+  // block.
+  // TODO: Find a common type, and convert both operands to it instead.
+  auto result_type_id = context.insts().Get(then_value_id).type_id();
+  else_value_id =
+      ConvertToValueOfType(context, else_node, else_value_id, result_type_id);
+
+  // Create a resumption block and branches to it.
+  auto chosen_value_id = context.AddConvergenceBlockWithArgAndPush(
+      if_node, {else_value_id, then_value_id});
+  context.AddCurrentCodeBlockToFunction(parse_node);
+
+  // Push the result value.
+  context.node_stack().Push(else_node, chosen_value_id);
+  return true;
+}
+
+}  // namespace Carbon::Check