[stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

src/mips64/interface-descriptors-mips64.cc

Index: src/mips64/interface-descriptors-mips64.cc
diff --git a/src/mips64/interface-descriptors-mips64.cc b/src/mips64/interface-descriptors-mips64.cc
index 42513e0866ecbf3085214f424fef6677550bbcb5..98a2821f86e0f522bb5eff33251cfe4dab66ccbf 100644
--- a/src/mips64/interface-descriptors-mips64.cc
+++ b/src/mips64/interface-descriptors-mips64.cc
@@ -50,6 +50,10 @@ const Register InstanceOfDescriptor::LeftRegister() { return a1; }
 const Register InstanceOfDescriptor::RightRegister() { return a0; }
 
 
+const Register StringCompareDescriptor::LeftRegister() { return a1; }
+const Register StringCompareDescriptor::RightRegister() { return a0; }
+
+
 const Register ArgumentsAccessReadDescriptor::index() { return a1; }
 const Register ArgumentsAccessReadDescriptor::parameter_count() { return a0; }