[PowerPC] Fix issue where hint was providing the incorrect regsiter class.

Regsier hints when copying to a UACC register do not always produce VSRp
registers. This patch makes sure that we do not produce hints in cases
where the subregsiter of the UACC is not a VSRp.

Reviewed By: nemanjai, #powerpc

Differential Revision: https://reviews.llvm.org/D107101