[turbofan] Introduce ObjectIsString operator.

src/compiler/change-lowering.h

Index: src/compiler/change-lowering.h
diff --git a/src/compiler/change-lowering.h b/src/compiler/change-lowering.h
index 7e5078bf84bed20e0fe8f7fa12bbc9a6821e0876..677b497f357f14cf0433d49e6f4973ec1cc59a05 100644
--- a/src/compiler/change-lowering.h
+++ b/src/compiler/change-lowering.h
@@ -64,6 +64,7 @@ class ChangeLowering final : public Reducer {
   Reduction ObjectIsNumber(Node* node);
   Reduction ObjectIsReceiver(Node* node);
   Reduction ObjectIsSmi(Node* node);
+  Reduction ObjectIsString(Node* node);
   Reduction ObjectIsUndetectable(Node* node);
 
   Node* ComputeIndex(const ElementAccess& access, Node* const key);