[turbofan] Enable typed lowering of string addition.

src/compiler/js-typed-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_JS_TYPED_LOWERING_H_
 #define V8_COMPILER_JS_TYPED_LOWERING_H_
 
 #include "src/compiler/graph-reducer.h"
 #include "src/compiler/opcodes.h"
 #include "src/compiler/simplified-operator.h"
@@ skipping 54 matching lines @@
   Reduction ReduceNumberBinop(Node* node, const Operator* numberOp);
   Reduction ReduceInt32Binop(Node* node, const Operator* intOp);
   Reduction ReduceUI32Shift(Node* node, Signedness left_signedness,
                             const Operator* shift_op);
 
   Node* Word32Shl(Node* const lhs, int32_t const rhs);
 
   Factory* factory() const;
   Graph* graph() const;
   JSGraph* jsgraph() const { return jsgraph_; }
+  Isolate* isolate() const;
   JSOperatorBuilder* javascript() const;
   CommonOperatorBuilder* common() const;
   SimplifiedOperatorBuilder* simplified() { return &simplified_; }
   MachineOperatorBuilder* machine() const;
 
   // Limits up to which context allocations are inlined.
   static const int kBlockContextAllocationLimit = 16;
 
   JSGraph* jsgraph_;
   SimplifiedOperatorBuilder simplified_;
   Type* zero_range_;
   Type* one_range_;
   Type* zero_thirtyone_range_;
   Type* shifted_int32_ranges_[4];
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_JS_TYPED_LOWERING_H_