[turbofan] Enable typed lowering of string addition.

test/cctest/compiler/simplified-graph-builder.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_CCTEST_COMPILER_SIMPLIFIED_GRAPH_BUILDER_H_
 #define V8_CCTEST_COMPILER_SIMPLIFIED_GRAPH_BUILDER_H_
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/graph-builder.h"
 #include "src/compiler/machine-operator.h"
@@ skipping 74 matching lines @@
 
   Node* StringEqual(Node* a, Node* b) {
     return NewNode(simplified()->StringEqual(), a, b);
   }
   Node* StringLessThan(Node* a, Node* b) {
     return NewNode(simplified()->StringLessThan(), a, b);
   }
   Node* StringLessThanOrEqual(Node* a, Node* b) {
     return NewNode(simplified()->StringLessThanOrEqual(), a, b);
   }
-  Node* StringAdd(Node* a, Node* b) {
-    return NewNode(simplified()->StringAdd(), a, b);
-  }
 
   Node* ChangeTaggedToInt32(Node* a) {
     return NewNode(simplified()->ChangeTaggedToInt32(), a);
   }
   Node* ChangeTaggedToUint32(Node* a) {
     return NewNode(simplified()->ChangeTaggedToUint32(), a);
   }
   Node* ChangeTaggedToFloat64(Node* a) {
     return NewNode(simplified()->ChangeTaggedToFloat64(), a);
   }
@@ skipping 37 matching lines @@
   CommonOperatorBuilder* common_;
   MachineOperatorBuilder* machine_;
   SimplifiedOperatorBuilder* simplified_;
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_CCTEST_COMPILER_SIMPLIFIED_GRAPH_BUILDER_H_