X86: Use LiveRegUnits in findDeadCallerSavedReg
My goal here was to eliminate the use of getGPRsForTailCall.
I don't think it makes sense to use here, considering this function
handles other non-tail call cases. It seems to have been used to
find a non-callee saved register in a roundabout way. We can be more
precise by letting LiveRegUnits figure out the CSR set for the current
function.
There are a few things I find confusing about this function; the API
isn't right. The callers ideally would be maintaining LiveRegUnits
in the context where they need the free register. It also doesn't
provide a RegisterClass to use. Also, this should work for any
block and this shouldn't need to special case this set of return
opcodes.
There are a few codegen test changes. One of them I think is a correct
improvement since the old code didn't consider undef uses as available.
The others I think are just to allocation order changes, since we're now
using the broader GPR_*NOSP classes.
