[PPC64] Use mfocrf in prologue when we only need to save 1 nonvolatile CR field

In the ELFv2 ABI, we are not required to save all CR fields. If only one
nonvolatile CR field is clobbered, use mfocrf instead of mfcr to
selectively save the field, because mfocrf has short latency compares to
mfcr.

Thanks Nemanja's invaluable hint!
Reviewers: nemanjai tjablin hfinkel kbarton

http://reviews.llvm.org/D17749

llvm-svn: 266038