Consistently use HStringAdd instead of HCallStub with CodeStub::StringAdd.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index e52dfff2ff6bc989703ec267ab8471809020bfbf..5186ce05b75585313d266493246a7dbacd94b577 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -921,11 +921,6 @@ void LCodeGen::DoCallStub(LCallStub* instr) {
       CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
       break;
     }
-    case CodeStub::StringAdd: {
-      StringAddStub stub(NO_STRING_ADD_FLAGS);
-      CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
-      break;
-    }
     case CodeStub::StringCompare: {
       StringCompareStub stub;
       CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
@@ -4324,7 +4319,7 @@ void LCodeGen::DoTrapAllocationMemento(LTrapAllocationMemento* instr) {
 void LCodeGen::DoStringAdd(LStringAdd* instr) {
   EmitPushTaggedOperand(instr->left());
   EmitPushTaggedOperand(instr->right());
-  StringAddStub stub(NO_STRING_CHECK_IN_STUB);
+  StringAddStub stub(instr->hydrogen()->flags());
   CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
 }