[turbofan] Add new StringFromCharCode simplified operator.

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 dfeb409291a9ba0090dae9e662404874ccf4df2e..7dc5141ca8267e75f10768d71ce2b4a07b6d91a3 100644
--- a/src/compiler/js-builtin-reducer.h
+++ b/src/compiler/js-builtin-reducer.h
@@ -40,6 +40,7 @@ class JSBuiltinReducer final : public AdvancedReducer {
   Reduction ReduceMathRound(Node* node);
   Reduction ReduceMathSqrt(Node* node);
   Reduction ReduceMathTrunc(Node* node);
+  Reduction ReduceStringFromCharCode(Node* node);
 
   Graph* graph() const;
   JSGraph* jsgraph() const { return jsgraph_; }