[turbofan] Enable typed lowering of string addition.

src/compiler/simplified-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_SIMPLIFIED_LOWERING_H_
 #define V8_COMPILER_SIMPLIFIED_LOWERING_H_
 
 #include "src/compiler/js-graph.h"
 #include "src/compiler/machine-operator.h"
 #include "src/compiler/node.h"
@@ skipping 20 matching lines @@
   void DoAllocate(Node* node);
   void DoLoadField(Node* node);
   void DoStoreField(Node* node);
   // TODO(turbofan): The output_type can be removed once the result of the
   // representation analysis is stored in the node bounds.
   void DoLoadBuffer(Node* node, MachineType output_type,
                     RepresentationChanger* changer);
   void DoStoreBuffer(Node* node);
   void DoLoadElement(Node* node);
   void DoStoreElement(Node* node);
-  void DoStringAdd(Node* node);
   void DoStringEqual(Node* node);
   void DoStringLessThan(Node* node);
   void DoStringLessThanOrEqual(Node* node);
 
  private:
   JSGraph* const jsgraph_;
   Zone* const zone_;
 
   // TODO(danno): SimplifiedLowering shouldn't know anything about the source
   // positions table, but must for now since there currently is no other way to
@@ skipping 20 matching lines @@
   Graph* graph() { return jsgraph()->graph(); }
   CommonOperatorBuilder* common() { return jsgraph()->common(); }
   MachineOperatorBuilder* machine() { return jsgraph()->machine(); }
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_SIMPLIFIED_LOWERING_H_