[turbofan] Enable typed lowering of string addition.

test/unittests/compiler/simplified-operator-unittest.cc

Index: test/unittests/compiler/simplified-operator-unittest.cc
diff --git a/test/unittests/compiler/simplified-operator-unittest.cc b/test/unittests/compiler/simplified-operator-unittest.cc
index a5dad5a4154e9a4cf4c1d47c034371c8e9548eb4..07728913b1807cd8b60e75d6d3e33fa626e9ed6b 100644
--- a/test/unittests/compiler/simplified-operator-unittest.cc
+++ b/test/unittests/compiler/simplified-operator-unittest.cc
@@ -54,7 +54,6 @@ const PureOperator kPureOperators[] = {
     PURE(StringEqual, Operator::kCommutative, 2),
     PURE(StringLessThan, Operator::kNoProperties, 2),
     PURE(StringLessThanOrEqual, Operator::kNoProperties, 2),
-    PURE(StringAdd, Operator::kNoProperties, 2),
     PURE(ChangeTaggedToInt32, Operator::kNoProperties, 1),
     PURE(ChangeTaggedToUint32, Operator::kNoProperties, 1),
     PURE(ChangeTaggedToFloat64, Operator::kNoProperties, 1),