[turbofan] Eliminate empty 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/base/compiler-specific.h"
 #include "src/base/flags.h"
 #include "src/compiler/graph-reducer.h"
@@ skipping 87 matching lines @@
   Isolate* isolate() const;
   JSOperatorBuilder* javascript() const;
   CommonOperatorBuilder* common() const;
   SimplifiedOperatorBuilder* simplified() const;
   CompilationDependencies* dependencies() const;
   Flags flags() const { return flags_; }
 
   CompilationDependencies* dependencies_;
   Flags flags_;
   JSGraph* jsgraph_;
+  Type* empty_string_type_;
   Type* shifted_int32_ranges_[4];
   Type* pointer_comparable_type_;
   TypeCache const& type_cache_;
 };
 
 DEFINE_OPERATORS_FOR_FLAGS(JSTypedLowering::Flags)
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_JS_TYPED_LOWERING_H_