[turbofan] Enable typed lowering of string addition.

src/compiler/simplified-lowering.cc

Index: src/compiler/simplified-lowering.cc
diff --git a/src/compiler/simplified-lowering.cc b/src/compiler/simplified-lowering.cc
index 929f8fdc405e2ba7fb45680d335cd2c7257cc43e..64f7e2249d4099a98414fa1a4c47a0fb5d62ed7b 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -782,11 +782,6 @@ class RepresentationSelector {
         if (lower()) lowering->DoStringLessThanOrEqual(node);
         break;
       }
-      case IrOpcode::kStringAdd: {
-        VisitBinop(node, kMachAnyTagged, kMachAnyTagged);
-        if (lower()) lowering->DoStringAdd(node);
-        break;
-      }
       case IrOpcode::kAllocate: {
         ProcessInput(node, 0, kMachAnyTagged);
         ProcessRemainingInputs(node, 1);
@@ -1313,23 +1308,6 @@ void SimplifiedLowering::DoStoreElement(Node* node) {
 }
 
 
-void SimplifiedLowering::DoStringAdd(Node* node) {
-  Operator::Properties properties = node->op()->properties();
-  Callable callable = CodeFactory::StringAdd(
-      jsgraph()->isolate(), STRING_ADD_CHECK_NONE, NOT_TENURED);
-  CallDescriptor::Flags flags = CallDescriptor::kNoFlags;
-  CallDescriptor* desc = Linkage::GetStubCallDescriptor(
-      jsgraph()->isolate(), zone(), callable.descriptor(), 0, flags,
-      properties);
-  node->set_op(common()->Call(desc));
-  node->InsertInput(graph()->zone(), 0,
-                    jsgraph()->HeapConstant(callable.code()));
-  node->AppendInput(graph()->zone(), jsgraph()->UndefinedConstant());
-  node->AppendInput(graph()->zone(), graph()->start());
-  node->AppendInput(graph()->zone(), graph()->start());
-}
-
-
 Node* SimplifiedLowering::StringComparison(Node* node, bool requires_ordering) {
   Runtime::FunctionId f =
       requires_ordering ? Runtime::kStringCompareRT : Runtime::kStringEquals;