[stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

src/x64/interface-descriptors-x64.cc

Index: src/x64/interface-descriptors-x64.cc
diff --git a/src/x64/interface-descriptors-x64.cc b/src/x64/interface-descriptors-x64.cc
index 146d9a89a56353291d60a129d200f9d409da56ea..ada386ffcbb46cc2e26051bda9672c1fffb5c870 100644
--- a/src/x64/interface-descriptors-x64.cc
+++ b/src/x64/interface-descriptors-x64.cc
@@ -50,6 +50,10 @@ const Register InstanceOfDescriptor::LeftRegister() { return rdx; }
 const Register InstanceOfDescriptor::RightRegister() { return rax; }
 
 
+const Register StringCompareDescriptor::LeftRegister() { return rdx; }
+const Register StringCompareDescriptor::RightRegister() { return rax; }
+
+
 const Register ArgumentsAccessReadDescriptor::index() { return rdx; }
 const Register ArgumentsAccessReadDescriptor::parameter_count() { return rax; }