[turbofan] Use a vector instead of a set in move optimizer.

src/compiler/move-optimizer.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_MOVE_OPTIMIZER_
 #define V8_COMPILER_MOVE_OPTIMIZER_
 
 #include "src/compiler/instruction.h"
 #include "src/globals.h"
 #include "src/zone/zone-containers.h"
@@ skipping 35 matching lines @@
   const Instruction* LastInstruction(const InstructionBlock* block) const;
 
   // Consolidate common moves appearing accross all predecessors of a block.
   void OptimizeMerge(InstructionBlock* block);
   void FinalizeMoves(Instruction* instr);
 
   Zone* const local_zone_;
   InstructionSequence* const code_;
   MoveOpVector local_vector_;
 
+  // Reusable buffers for storing operand sets. We need at most two sets
+  // at any given time, so we create two buffers.
+  ZoneVector<InstructionOperand> operand_buffer1;
+  ZoneVector<InstructionOperand> operand_buffer2;
+
   DISALLOW_COPY_AND_ASSIGN(MoveOptimizer);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_MOVE_OPTIMIZER_