[turbofan] Enable typed lowering of string addition.

src/compiler/typer.cc

 // 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.
 
 #include "src/base/flags.h"
 #include "src/bootstrapper.h"
 #include "src/compiler/graph-reducer.h"
 #include "src/compiler/js-operator.h"
 #include "src/compiler/node.h"
 #include "src/compiler/node-properties.h"
@@ skipping 1711 matching lines @@
 Bounds Typer::Visitor::TypeStringLessThan(Node* node) {
   return Bounds(Type::None(zone()), Type::Boolean(zone()));
 }
 
 
 Bounds Typer::Visitor::TypeStringLessThanOrEqual(Node* node) {
   return Bounds(Type::None(zone()), Type::Boolean(zone()));
 }
 
 
-Bounds Typer::Visitor::TypeStringAdd(Node* node) {
-  return Bounds(Type::None(zone()), Type::String(zone()));
-}
-
-
 namespace {
 
 Type* ChangeRepresentation(Type* type, Type* rep, Zone* zone) {
   return Type::Union(Type::Semantic(type, zone),
                      Type::Representation(rep, zone), zone);
 }
 
 }  // namespace
 
 
@@ skipping 653 matching lines @@
       TYPED_ARRAYS(TYPED_ARRAY_CASE)
 #undef TYPED_ARRAY_CASE
     }
   }
   return Type::Constant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8