[stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

src/arm64/interface-descriptors-arm64.cc

Index: src/arm64/interface-descriptors-arm64.cc
diff --git a/src/arm64/interface-descriptors-arm64.cc b/src/arm64/interface-descriptors-arm64.cc
index 76bcca3e57ecc08ca59181ff31e40b1380ec133a..5c117025c78098749def4086d9c71c70683a8f7e 100644
--- a/src/arm64/interface-descriptors-arm64.cc
+++ b/src/arm64/interface-descriptors-arm64.cc
@@ -52,6 +52,10 @@ const Register InstanceOfDescriptor::LeftRegister() { return x1; }
 const Register InstanceOfDescriptor::RightRegister() { return x0; }
 
 
+const Register StringCompareDescriptor::LeftRegister() { return x1; }
+const Register StringCompareDescriptor::RightRegister() { return x0; }
+
+
 const Register ArgumentsAccessReadDescriptor::index() { return x1; }
 const Register ArgumentsAccessReadDescriptor::parameter_count() { return x0; }