[compiler] Introduce StringEqualStub and StringNotEqualStub.

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

Index: src/crankshaft/mips/lithium-codegen-mips.cc
diff --git a/src/crankshaft/mips/lithium-codegen-mips.cc b/src/crankshaft/mips/lithium-codegen-mips.cc
index 1073f86af0a28d9f7602cd1352962cbfee3d917e..dd908dea5ca3c9c15ee965c831148091bc1bfcfc 100644
--- a/src/crankshaft/mips/lithium-codegen-mips.cc
+++ b/src/crankshaft/mips/lithium-codegen-mips.cc
@@ -2275,11 +2275,19 @@ void LCodeGen::DoStringCompareAndBranch(LStringCompareAndBranch* instr) {
   DCHECK(ToRegister(instr->left()).is(a1));
   DCHECK(ToRegister(instr->right()).is(a0));
 
-  Handle<Code> code = CodeFactory::StringCompare(isolate()).code();
-  CallCode(code, RelocInfo::CODE_TARGET, instr);
+  if (Token::IsOrderedRelationalCompareOp(instr->op())) {
+    Handle<Code> code = CodeFactory::StringCompare(isolate()).code();
+    CallCode(code, RelocInfo::CODE_TARGET, instr);
+
+    EmitBranch(instr, ComputeCompareCondition(instr->op()), v0,
+               Operand(zero_reg));
+  } else {
+    Handle<Code> code = CodeFactory::StringEqual(isolate()).code();
+    CallCode(code, RelocInfo::CODE_TARGET, instr);
+    __ LoadRoot(at, Heap::kTrueValueRootIndex);
 
-  EmitBranch(instr, ComputeCompareCondition(instr->op()), v0,
-             Operand(zero_reg));
+    EmitBranch(instr, ComputeCompareCondition(instr->op()), v0, Operand(at));
+  }
 }