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

We currently use BitcastTaggedToWord only in from the code assemblers to verify the correctness of the operation.

BUG=

Review-Url: https://codereview.chromium.org/2605073002
Cr-Commit-Position: refs/heads/master@{#41979}

src/compiler/instruction-selector.cc

@@ -1636,8 +1636,7 @@ void InstructionSelector::VisitStackSlot(Node* node) {
 }
 
 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) {