[compiler] Introduce StringEqualStub and StringNotEqualStub.

src/crankshaft/x64/lithium-codegen-x64.cc

Index: src/crankshaft/x64/lithium-codegen-x64.cc
diff --git a/src/crankshaft/x64/lithium-codegen-x64.cc b/src/crankshaft/x64/lithium-codegen-x64.cc
index 35855179df2f4e169301006916848f0ce8749069..f8dd6662d315a27e1e2c2fecd00e9663627bd7b5 100644
--- a/src/crankshaft/x64/lithium-codegen-x64.cc
+++ b/src/crankshaft/x64/lithium-codegen-x64.cc
@@ -2318,9 +2318,15 @@ void LCodeGen::DoStringCompareAndBranch(LStringCompareAndBranch* instr) {
   DCHECK(ToRegister(instr->left()).is(rdx));
   DCHECK(ToRegister(instr->right()).is(rax));
 
-  Handle<Code> code = CodeFactory::StringCompare(isolate()).code();
-  CallCode(code, RelocInfo::CODE_TARGET, instr);
-  __ testp(rax, rax);
+  if (Token::IsOrderedRelationalCompareOp(instr->op())) {
+    Handle<Code> code = CodeFactory::StringCompare(isolate()).code();
+    CallCode(code, RelocInfo::CODE_TARGET, instr);
+    __ testp(rax, rax);
+  } else {
+    Handle<Code> code = CodeFactory::StringEqual(isolate()).code();
+    CallCode(code, RelocInfo::CODE_TARGET, instr);
+    __ CompareRoot(rax, Heap::kTrueValueRootIndex);
+  }
 
   EmitBranch(instr, TokenToCondition(instr->op(), false));
 }