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

src/compiler/js-graph.h

Index: src/compiler/js-graph.h
diff --git a/src/compiler/js-graph.h b/src/compiler/js-graph.h
index fe5545a04d8d5f2096b06d8c38104018e906ffec..87e01f639807764e87c5565f1437f1cb5ff0f18b 100644
--- a/src/compiler/js-graph.h
+++ b/src/compiler/js-graph.h
@@ -45,6 +45,7 @@ class JSGraph : public ZoneObject {
   Node* CEntryStubConstant(int result_size);
   Node* EmptyFixedArrayConstant();
   Node* EmptyLiteralsArrayConstant();
+  Node* EmptyStringConstant();
   Node* HeapNumberMapConstant();
   Node* OptimizedOutConstant();
   Node* StaleRegisterConstant();
@@ -151,6 +152,7 @@ class JSGraph : public ZoneObject {
     kCEntryStubConstant,
     kEmptyFixedArrayConstant,
     kEmptyLiteralsArrayConstant,
+    kEmptyStringConstant,
     kHeapNumberMapConstant,
     kOptimizedOutConstant,
     kStaleRegisterConstant,