[turbofan] Introduce separate SelectLowering reducer.

src/compiler/select-lowering.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_SELECT_LOWERING_H_
+#define V8_COMPILER_SELECT_LOWERING_H_
+
+#include <map>
+
+#include "src/compiler/graph-reducer.h"
+#include "src/zone-allocator.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+// Forward declarations.
+class CommonOperatorBuilder;
+class Graph;
+
+
+// Lowers Select nodes to diamonds.
+class SelectLowering FINAL : public Reducer {
+ public:
+  SelectLowering(Graph* graph, CommonOperatorBuilder* common);
+  ~SelectLowering();
+
+  Reduction Reduce(Node* node) OVERRIDE;
+
+ private:
+  typedef std::map<Node*, Node*, std::less<Node*>,
+                   zone_allocator<std::pair<Node* const, Node*>>> Merges;
+
+  CommonOperatorBuilder* common() const { return common_; }
+  Graph* graph() const { return graph_; }
+
+  CommonOperatorBuilder* common_;
+  Graph* graph_;
+  Merges merges_;
+};
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_COMPILER_SELECT_LOWERING_H_