Add a RM pseudoreg for the rounding mode, which

allows ppcf128->int conversion to work with
DeadInstructionElimination.  This is now turned
off but RM is harmless.  It does not do a complete
job of modeling the rounding mode.

Revert marking MFCR as using all 7 CR subregisters;
while correct, this caused the problem in PR 2964,
plus the local RA crash noted in the comments.
This was needed to make DeadInstructionElimination,
but as we are not running that, it is backed out
for now.  Eventually it should go back in and the
other problems fixed where they're broken.

llvm-svn: 58391