[turbofan] Enable typed lowering of string addition.

test/cctest/compiler/simplified-graph-builder.h

Index: test/cctest/compiler/simplified-graph-builder.h
diff --git a/test/cctest/compiler/simplified-graph-builder.h b/test/cctest/compiler/simplified-graph-builder.h
index c9ba002c258535757224c5253aa6de567b7490f8..50c51d5ed840bf1aa91f6ac880a2c69f27902eb5 100644
--- a/test/cctest/compiler/simplified-graph-builder.h
+++ b/test/cctest/compiler/simplified-graph-builder.h
@@ -92,9 +92,6 @@ class SimplifiedGraphBuilder : public GraphBuilder {
   Node* StringLessThanOrEqual(Node* a, Node* b) {
     return NewNode(simplified()->StringLessThanOrEqual(), a, b);
   }
-  Node* StringAdd(Node* a, Node* b) {
-    return NewNode(simplified()->StringAdd(), a, b);
-  }
 
   Node* ChangeTaggedToInt32(Node* a) {
     return NewNode(simplified()->ChangeTaggedToInt32(), a);