[stubs] Refactor StringCompareStub and use it for HStringCompareAndBranch.

src/mips/interface-descriptors-mips.cc

Index: src/mips/interface-descriptors-mips.cc
diff --git a/src/mips/interface-descriptors-mips.cc b/src/mips/interface-descriptors-mips.cc
index 3e566293ae1288d5dff42eb99dee1f4ba56fb5a1..06bf028c6cbb42925c84973efa81d03c7df169ae 100644
--- a/src/mips/interface-descriptors-mips.cc
+++ b/src/mips/interface-descriptors-mips.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; }