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

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
diff --git a/src/ia32/lithium-codegen-ia32.cc b/src/ia32/lithium-codegen-ia32.cc
index 54fb1877da23260b578664b08fd2e1e0ee2f7b26..77d166faa3612c8576b7d5d57f7d91386f56fae4 100644
--- a/src/ia32/lithium-codegen-ia32.cc
+++ b/src/ia32/lithium-codegen-ia32.cc
@@ -1222,11 +1222,6 @@ 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);
@@ -4873,7 +4868,7 @@ void LCodeGen::DoStringLength(LStringLength* 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);
 }