[Turbofan] Fix IsSlot function in MoveOptimizer.

LOG=N
BUG=v8:4124

Review-Url: https://codereview.chromium.org/2328423002
Cr-Commit-Position: refs/heads/master@{#39389}

src/compiler/move-optimizer.cc

@@ -424,7 +424,7 @@ void MoveOptimizer::OptimizeMerge(InstructionBlock* block) {
 namespace {
 
 bool IsSlot(const InstructionOperand& op) {
-  return op.IsStackSlot() || op.IsDoubleStackSlot();
+  return op.IsStackSlot() || op.IsFPStackSlot();
 }