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

src/mips/lithium-codegen-mips.cc

Index: src/mips/lithium-codegen-mips.cc
diff --git a/src/mips/lithium-codegen-mips.cc b/src/mips/lithium-codegen-mips.cc
index 2ee70ff45e3e42dbbdaf77f87e0ef6e8ae573003..f16266468f76b72d724e421e1c035b2e2d9ab7e1 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -1024,11 +1024,6 @@ 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);
@@ -4478,7 +4473,7 @@ void LCodeGen::DoTrapAllocationMemento(LTrapAllocationMemento* instr) {
 void LCodeGen::DoStringAdd(LStringAdd* instr) {
   __ push(ToRegister(instr->left()));
   __ push(ToRegister(instr->right()));
-  StringAddStub stub(NO_STRING_CHECK_IN_STUB);
+  StringAddStub stub(instr->hydrogen()->flags());
   CallCode(stub.GetCode(isolate()), RelocInfo::CODE_TARGET, instr);
 }