[turbofan] Introduce ObjectIsString operator.

src/compiler/simplified-operator.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_SIMPLIFIED_OPERATOR_H_
 #define V8_COMPILER_SIMPLIFIED_OPERATOR_H_
 
 #include <iosfwd>
 
 #include "src/handles.h"
@@ skipping 153 matching lines @@
   const Operator* ChangeTaggedToFloat64();
   const Operator* ChangeInt32ToTagged();
   const Operator* ChangeUint32ToTagged();
   const Operator* ChangeFloat64ToTagged();
   const Operator* ChangeBoolToBit();
   const Operator* ChangeBitToBool();
 
   const Operator* ObjectIsNumber();
   const Operator* ObjectIsReceiver();
   const Operator* ObjectIsSmi();
+  const Operator* ObjectIsString();
   const Operator* ObjectIsUndetectable();
 
   const Operator* Allocate(PretenureFlag pretenure = NOT_TENURED);
 
   const Operator* LoadField(FieldAccess const&);
   const Operator* StoreField(FieldAccess const&);
 
   // load-buffer buffer, offset, length
   const Operator* LoadBuffer(BufferAccess);
 
@@ skipping 13 matching lines @@
   Zone* const zone_;
 
   DISALLOW_COPY_AND_ASSIGN(SimplifiedOperatorBuilder);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_SIMPLIFIED_OPERATOR_H_