[turbofan] Enable typed lowering of string addition.

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/machine-type.h"
@@ skipping 131 matching lines @@
   const Operator* NumberToInt32();
   const Operator* NumberToUint32();
 
   const Operator* PlainPrimitiveToNumber();
 
   const Operator* ReferenceEqual(Type* type);
 
   const Operator* StringEqual();
   const Operator* StringLessThan();
   const Operator* StringLessThanOrEqual();
-  const Operator* StringAdd();
 
   const Operator* ChangeTaggedToInt32();
   const Operator* ChangeTaggedToUint32();
   const Operator* ChangeTaggedToFloat64();
   const Operator* ChangeInt32ToTagged();
   const Operator* ChangeUint32ToTagged();
   const Operator* ChangeFloat64ToTagged();
   const Operator* ChangeBoolToBit();
   const Operator* ChangeBitToBool();
 
@@ skipping 24 matching lines @@
   Zone* const zone_;
 
   DISALLOW_COPY_AND_ASSIGN(SimplifiedOperatorBuilder);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_SIMPLIFIED_OPERATOR_H_