[turbofan] Add support for String.prototype.charCodeAt/charAt.

src/compiler/js-graph.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/code-stubs.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/node-properties.h"
 #include "src/compiler/typer.h"
 
 namespace v8 {
@@ skipping 30 matching lines @@
 Node* JSGraph::EmptyFixedArrayConstant() {
   return CACHED(kEmptyFixedArrayConstant,
                 HeapConstant(factory()->empty_fixed_array()));
 }
 
 Node* JSGraph::EmptyLiteralsArrayConstant() {
   return CACHED(kEmptyLiteralsArrayConstant,
                 HeapConstant(factory()->empty_literals_array()));
 }
 
+Node* JSGraph::EmptyStringConstant() {
+  return CACHED(kEmptyStringConstant, HeapConstant(factory()->empty_string()));
+}
+
 Node* JSGraph::HeapNumberMapConstant() {
   return CACHED(kHeapNumberMapConstant,
                 HeapConstant(factory()->heap_number_map()));
 }
 
 Node* JSGraph::OptimizedOutConstant() {
   return CACHED(kOptimizedOutConstant,
                 HeapConstant(factory()->optimized_out()));
 }
 
@@ skipping 182 matching lines @@
   for (size_t i = 0; i < arraysize(cached_nodes_); i++) {
     if (Node* node = cached_nodes_[i]) {
       if (!node->IsDead()) nodes->push_back(node);
     }
   }
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8