[turbofan] Introduce dedicated BitcastWordToTagged machine operator.

src/compiler/machine-operator.h

Index: src/compiler/machine-operator.h
diff --git a/src/compiler/machine-operator.h b/src/compiler/machine-operator.h
index 8e43f48042ed18456e5c47ffe07914729b039e1c..d8861e0c5e49a16f3961aa4f6bc928f4cdb44c77 100644
--- a/src/compiler/machine-operator.h
+++ b/src/compiler/machine-operator.h
@@ -220,6 +220,9 @@ class MachineOperatorBuilder final : public ZoneObject {
   const Operator* Uint64LessThanOrEqual();
   const Operator* Uint64Mod();
 
+  // This operator reinterprets the bits of a word as tagged pointer.
+  const Operator* BitcastWordToTagged();
+
   // These operators change the representation of numbers while preserving the
   // value of the number. Narrowing operators assume the input is representable
   // in the target type and are *not* defined for other inputs.