[compiler] Introduce a simple store-store elimination, disabled by default.

src/compiler/store-store-elimination.h

+// Copyright 2016 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_STORE_STORE_ELIMINATION_H_
+#define V8_COMPILER_STORE_STORE_ELIMINATION_H_
+
+#include "src/compiler/common-operator.h"
+#include "src/compiler/graph-reducer.h"
+#include "src/compiler/js-graph.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+// Forward declarations.
+class CommonOperatorBuilder;
+class JSGraph;
+
+class StoreStoreElimination final {
+ public:
+  StoreStoreElimination(JSGraph* js_graph, Zone* temp_zone);
+  ~StoreStoreElimination();
+  void Run();
+
+ private:
+  static bool IsEligibleNode(Node* node);
+  void ReduceEligibleNode(Node* node);
+  JSGraph* jsgraph() const { return jsgraph_; }
+  Zone* temp_zone() const { return temp_zone_; }
+
+  JSGraph* const jsgraph_;
+  Zone* const temp_zone_;
+};
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_COMPILER_STORE_STORE_ELIMINATION_H_