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

src/compiler/js-builtin-reducer.h

Index: src/compiler/js-builtin-reducer.h
diff --git a/src/compiler/js-builtin-reducer.h b/src/compiler/js-builtin-reducer.h
index a3d3ee54b4767f66c030da7833e59f88dd626697..e9c49e4a802bae07d6998fa2bb5493d659a854e9 100644
--- a/src/compiler/js-builtin-reducer.h
+++ b/src/compiler/js-builtin-reducer.h
@@ -63,6 +63,8 @@ class JSBuiltinReducer final : public AdvancedReducer {
   Reduction ReduceMathTanh(Node* node);
   Reduction ReduceMathTrunc(Node* node);
   Reduction ReduceNumberParseInt(Node* node);
+  Reduction ReduceStringCharAt(Node* node);
+  Reduction ReduceStringCharCodeAt(Node* node);
   Reduction ReduceStringFromCharCode(Node* node);
 
   Node* ToNumber(Node* value);