Swiftself uses a callee-saved register. We can tail call when the register used in the caller and callee is the same.
This behaviour is equivalent to that in TargetLowering::parametersInCSRMatch.
Update call-translator-tail-call.ll to verify that we can do this. When we support inline assembly, we can write a check similar to the one in the general swiftself.ll. For now, we need to verify that we get the correct COPY instruction after call lowering.