[PowerPC] Change VSRpRC allocation order

On PowerPC, VSRpRC represents the pairs of even and odd VSX register,
and VRRC corresponds to higher 32 VSX registers. In some cases, extra
copies are produced when handling incoming VRRC arguments with VSRpRC.

This patch changes allocation order of VSRpRC to eliminate this kind of
copy.

Stack frame sizes may increase if allocating non-volatile registers, and
some other vector copies happen. They need fix in future changes.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D104855