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

src/compiler/move-optimizer.h

Index: src/compiler/move-optimizer.h
diff --git a/src/compiler/move-optimizer.h b/src/compiler/move-optimizer.h
index ab7144e5b8f675609712805026464149131d3f1f..3844d330ec5f5982186b3256cb0ae42fbc5f7cda 100644
--- a/src/compiler/move-optimizer.h
+++ b/src/compiler/move-optimizer.h
@@ -53,6 +53,11 @@ class V8_EXPORT_PRIVATE MoveOptimizer final {
   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);
 };