[turbofan] Add support for pushing returns into merges.

src/compiler/common-operator-reducer.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COMPILER_COMMON_OPERATOR_REDUCER_H_
 #define V8_COMPILER_COMMON_OPERATOR_REDUCER_H_
 
 #include "src/compiler/graph-reducer.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
 
 // Forward declarations.
 class CommonOperatorBuilder;
 class Graph;
 class MachineOperatorBuilder;
 class Operator;
 
 
 // Performs strength reduction on nodes that have common operators.
 class CommonOperatorReducer final : public AdvancedReducer {
  public:
   CommonOperatorReducer(Editor* editor, Graph* graph,
                         CommonOperatorBuilder* common,
-                        MachineOperatorBuilder* machine)
-      : AdvancedReducer(editor),
-        graph_(graph),
-        common_(common),
-        machine_(machine) {}
+                        MachineOperatorBuilder* machine);
   ~CommonOperatorReducer() final {}
 
   Reduction Reduce(Node* node) final;
 
  private:
   Reduction ReduceBranch(Node* node);
   Reduction ReduceMerge(Node* node);
   Reduction ReduceEffectPhi(Node* node);
   Reduction ReducePhi(Node* node);
+  Reduction ReduceReturn(Node* node);
   Reduction ReduceSelect(Node* node);
 
   Reduction Change(Node* node, Operator const* op, Node* a);
   Reduction Change(Node* node, Operator const* op, Node* a, Node* b);
 
   Graph* graph() const { return graph_; }
   CommonOperatorBuilder* common() const { return common_; }
   MachineOperatorBuilder* machine() const { return machine_; }
+  Node* dead() const { return dead_; }
 
   Graph* const graph_;
   CommonOperatorBuilder* const common_;
   MachineOperatorBuilder* const machine_;
-  SetOncePointer<Node> dead_;
+  Node* const dead_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_COMMON_OPERATOR_REDUCER_H_