[turbofan] Introduce ObjectIsString operator.

src/compiler/change-lowering.h

 // 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.
 
 #ifndef V8_COMPILER_CHANGE_LOWERING_H_
 #define V8_COMPILER_CHANGE_LOWERING_H_
 
 #include "src/compiler/graph-reducer.h"
 
 namespace v8 {
@@ skipping 46 matching lines @@
   Reduction Allocate(Node* node);
 
   Node* IsSmi(Node* value);
   Node* LoadHeapObjectMap(Node* object, Node* control);
   Node* LoadMapBitField(Node* map);
   Node* LoadMapInstanceType(Node* map);
 
   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);
   Graph* graph() const;
   Isolate* isolate() const;
   JSGraph* jsgraph() const { return jsgraph_; }
   CommonOperatorBuilder* common() const;
   MachineOperatorBuilder* machine() const;
 
   JSGraph* const jsgraph_;
   SetOncePointer<const Operator> allocate_heap_number_operator_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_CHANGE_LOWERING_H_