[turbofan] Introduce dedicated BitcastWordToTagged machine operator.

src/compiler/instruction-selector.cc

Index: src/compiler/instruction-selector.cc
diff --git a/src/compiler/instruction-selector.cc b/src/compiler/instruction-selector.cc
index c18c474c345394ec576588b0eb16d6a1d26589c6..af6196736b36f50c468d5be7185bab0c068c3631 100644
--- a/src/compiler/instruction-selector.cc
+++ b/src/compiler/instruction-selector.cc
@@ -1014,6 +1014,8 @@ void InstructionSelector::VisitNode(Node* node) {
       return VisitUint64LessThanOrEqual(node);
     case IrOpcode::kUint64Mod:
       return MarkAsWord64(node), VisitUint64Mod(node);
+    case IrOpcode::kBitcastWordToTagged:
+      return MarkAsReference(node), VisitBitcastWordToTagged(node);
     case IrOpcode::kChangeFloat32ToFloat64:
       return MarkAsFloat64(node), VisitChangeFloat32ToFloat64(node);
     case IrOpcode::kChangeInt32ToFloat64:
@@ -1253,6 +1255,12 @@ void InstructionSelector::VisitStackSlot(Node* node) {
        sequence()->AddImmediate(Constant(slot)), 0, nullptr);
 }
 
+void InstructionSelector::VisitBitcastWordToTagged(Node* node) {
+  OperandGenerator g(this);
+  Node* value = node->InputAt(0);
+  Emit(kArchNop, g.DefineSameAsFirst(node), g.Use(value));
+}
+
 // 32 bit targets do not implement the following instructions.
 #if V8_TARGET_ARCH_32_BIT