PPC: [stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

src/ppc/interface-descriptors-ppc.cc

Index: src/ppc/interface-descriptors-ppc.cc
diff --git a/src/ppc/interface-descriptors-ppc.cc b/src/ppc/interface-descriptors-ppc.cc
index 9b9d1f6fcb50c3238853fc77e32ee47d6ac90b31..b6f96e65863bd8838e77866e528a0f82d2ec0c9a 100644
--- a/src/ppc/interface-descriptors-ppc.cc
+++ b/src/ppc/interface-descriptors-ppc.cc
@@ -50,6 +50,10 @@ const Register InstanceOfDescriptor::LeftRegister() { return r4; }
 const Register InstanceOfDescriptor::RightRegister() { return r3; }
 
 
+const Register StringCompareDescriptor::LeftRegister() { return r4; }
+const Register StringCompareDescriptor::RightRegister() { return r3; }
+
+
 const Register ArgumentsAccessReadDescriptor::index() { return r4; }
 const Register ArgumentsAccessReadDescriptor::parameter_count() { return r3; }