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

src/compiler/js-graph.cc

Index: src/compiler/js-graph.cc
diff --git a/src/compiler/js-graph.cc b/src/compiler/js-graph.cc
index 82c22c3719a0e683fe95214288734305275fd010..0af29d5475351e489592f8c0e6b889ad626a70a2 100644
--- a/src/compiler/js-graph.cc
+++ b/src/compiler/js-graph.cc
@@ -48,6 +48,10 @@ Node* JSGraph::EmptyLiteralsArrayConstant() {
                 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()));