[stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

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 7ef838ab10f5f127d63e8e310adf59be7a0bb9c8..81d275f7a9d60bb7dc6cc5ab3291c332cd3d33b4 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -999,11 +999,6 @@ void LCodeGen::DoCallStub(LCallStub* instr) {
       CallCode(stub.GetCode(), RelocInfo::CODE_TARGET, instr);
       break;
     }
-    case CodeStub::StringCompare: {
-      StringCompareStub stub(isolate());
-      CallCode(stub.GetCode(), RelocInfo::CODE_TARGET, instr);
-      break;
-    }
     default:
       UNREACHABLE();
   }
@@ -2491,16 +2486,14 @@ void LCodeGen::DoIsUndetectableAndBranch(LIsUndetectableAndBranch* instr) {
 
 void LCodeGen::DoStringCompareAndBranch(LStringCompareAndBranch* instr) {
   DCHECK(ToRegister(instr->context()).is(rsi));
-  Token::Value op = instr->op();
-
-  Handle<Code> ic =
-      CodeFactory::CompareIC(isolate(), op, Strength::WEAK).code();
-  CallCode(ic, RelocInfo::CODE_TARGET, instr);
+  DCHECK(ToRegister(instr->left()).is(rdx));
+  DCHECK(ToRegister(instr->right()).is(rax));
 
-  Condition condition = TokenToCondition(op, false);
+  Handle<Code> code = CodeFactory::StringCompare(isolate()).code();
+  CallCode(code, RelocInfo::CODE_TARGET, instr);
   __ testp(rax, rax);
 
-  EmitBranch(instr, condition);
+  EmitBranch(instr, TokenToCondition(instr->op(), false));
 }