[turbofan] Inline String.prototype.concat with PlainPrimitive parameter.

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 9dcc8b36c0e3678fefa945856e655695732447a4..e792ad3c3afeb7bcb7dffe54c6736412200ee4a7 100644
--- a/src/compiler/js-builtin-reducer.h
+++ b/src/compiler/js-builtin-reducer.h
@@ -103,6 +103,7 @@ class V8_EXPORT_PRIVATE JSBuiltinReducer final
   Reduction ReduceObjectCreate(Node* node);
   Reduction ReduceStringCharAt(Node* node);
   Reduction ReduceStringCharCodeAt(Node* node);
+  Reduction ReduceStringConcat(Node* node);
   Reduction ReduceStringFromCharCode(Node* node);
   Reduction ReduceStringIndexOf(Node* node);
   Reduction ReduceStringIterator(Node* node);