Split statement-specific parts of semantics_handle.cpp into separate files. (#2949)

`semantics_handle.cpp` is a little on the large side, and is going to grow as we add new nodes. Some of the statement-specific parts have already been split into their own files. Split out the remaining such parts.

toolchain/semantics/semantics_handle_if_statement.cpp

@@ -0,0 +1,86 @@
+// 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/semantics/semantics_context.h"
+#include "toolchain/semantics/semantics_node.h"
+
+namespace Carbon {
+
+auto SemanticsHandleIfConditionStart(SemanticsContext& /*context*/,
+                                     ParseTree::Node /*parse_node*/) -> bool {
+  return true;
+}
+
+auto SemanticsHandleIfCondition(SemanticsContext& context,
+                                ParseTree::Node parse_node) -> bool {
+  // Convert the condition to `bool`.
+  auto cond_value_id = context.node_stack().Pop<SemanticsNodeId>();
+  cond_value_id = context.ImplicitAsBool(parse_node, cond_value_id);
+
+  // Create the then block and the else block, and branch to the right one. If
+  // there is no `else`, the then block will terminate with a branch to the
+  // else block, which will be reused as the resumption block.
+  auto then_block_id =
+      context.AddDominatedBlockAndBranchIf(parse_node, cond_value_id);
+  auto else_block_id = context.AddDominatedBlockAndBranch(parse_node);
+
+  // Push the else and then blocks, and start emitting code in the then block.
+  context.node_block_stack().Pop();
+  context.node_block_stack().Push(else_block_id);
+  context.node_block_stack().Push(then_block_id);
+  context.AddCurrentCodeBlockToFunction();
+
+  context.node_stack().Push(parse_node);
+  return true;
+}
+
+auto SemanticsHandleIfStatementElse(SemanticsContext& context,
+                                    ParseTree::Node parse_node) -> bool {
+  context.node_stack().PopAndDiscardSoloParseNode(ParseNodeKind::IfCondition);
+
+  // Switch to emitting the else block.
+  auto then_block_id = context.node_block_stack().PopForAdd();
+  context.node_stack().Push(parse_node, then_block_id);
+  context.AddCurrentCodeBlockToFunction();
+  return true;
+}
+
+auto SemanticsHandleIfStatement(SemanticsContext& context,
+                                ParseTree::Node parse_node) -> bool {
+  // Either the then or else block, depending on whether there's an `else` node
+  // on the top of the node stack.
+  auto sub_block_id = context.node_block_stack().PopForAdd();
+
+  switch (auto kind = context.parse_tree().node_kind(
+              context.node_stack().PeekParseNode())) {
+    case ParseNodeKind::IfCondition: {
+      // Branch from then block to else block.
+      context.node_stack().PopAndDiscardSoloParseNode(
+          ParseNodeKind::IfCondition);
+      context.AddNodeToBlock(
+          sub_block_id,
+          SemanticsNode::Branch::Make(parse_node,
+                                      context.node_block_stack().PeekForAdd()));
+      break;
+    }
+
+    case ParseNodeKind::IfStatementElse: {
+      // Branch from the then and else blocks to a new resumption block.
+      auto then_block_id = context.node_stack().Pop<SemanticsNodeBlockId>(
+          ParseNodeKind::IfStatementElse);
+      context.AddConvergenceBlockAndPush(parse_node,
+                                         {then_block_id, sub_block_id});
+      break;
+    }
+
+    default: {
+      CARBON_FATAL() << "Unexpected parse node at start of `if`: " << kind;
+    }
+  }
+
+  context.AddCurrentCodeBlockToFunction();
+  return true;
+}
+
+}  // namespace Carbon