[turbofan] Introduce CheckString simplified 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/compiler/operator.h"
@@ skipping 255 matching lines @@
   const Operator* ChangeUint32ToTagged();
   const Operator* ChangeFloat64ToTagged();
   const Operator* ChangeTaggedToBit();
   const Operator* ChangeBitToTagged();
   const Operator* TruncateTaggedToWord32();
   const Operator* TruncateTaggedToFloat64();
 
   const Operator* CheckIf();
   const Operator* CheckBounds();
   const Operator* CheckNumber();
+  const Operator* CheckString();
   const Operator* CheckTaggedPointer();
   const Operator* CheckTaggedSigned();
 
   const Operator* CheckedInt32Add();
   const Operator* CheckedInt32Sub();
   const Operator* CheckedInt32Div();
   const Operator* CheckedInt32Mod();
   const Operator* CheckedUint32Div();
   const Operator* CheckedUint32Mod();
   const Operator* CheckedInt32Mul(CheckForMinusZeroMode);
@@ skipping 39 matching lines @@
   Zone* const zone_;
 
   DISALLOW_COPY_AND_ASSIGN(SimplifiedOperatorBuilder);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_SIMPLIFIED_OPERATOR_H_