[turbofan] Do not generate unnecessary moves when casting a tagged value to a word.

src/compiler/instruction-selector.cc

 // 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.
 
 #include "src/compiler/instruction-selector.h"
 
 #include <limits>
 
 #include "src/base/adapters.h"
 #include "src/compiler/compiler-source-position-table.h"
@@ skipping 1618 matching lines @@
 void InstructionSelector::VisitStackSlot(Node* node) {
   int size = 1 << ElementSizeLog2Of(StackSlotRepresentationOf(node->op()));
   int slot = frame_->AllocateSpillSlot(size);
   OperandGenerator g(this);
 
   Emit(kArchStackSlot, g.DefineAsRegister(node),
        sequence()->AddImmediate(Constant(slot)), 0, nullptr);
 }
 
 void InstructionSelector::VisitBitcastTaggedToWord(Node* node) {
-  OperandGenerator g(this);
-  Emit(kArchNop, g.DefineSameAsFirst(node), g.Use(node->InputAt(0)));
+  EmitIdentity(node);
 }
 
 void InstructionSelector::VisitBitcastWordToTagged(Node* node) {
   OperandGenerator g(this);
   Emit(kArchNop, g.DefineSameAsFirst(node), g.Use(node->InputAt(0)));
 }
 
 // 32 bit targets do not implement the following instructions.
 #if V8_TARGET_ARCH_32_BIT
 
@@ skipping 606 matching lines @@
   return new (instruction_zone()) FrameStateDescriptor(
       instruction_zone(), state_info.type(), state_info.bailout_id(),
       state_info.state_combine(), parameters, locals, stack,
       state_info.shared_info(), outer_state);
 }
 
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8