[PowerPC] Exploit xxperm, check for dead vectors and substitute vperm with xxperm

vperm instruction requires the data to be in the Altivec registers, if one of
the vector operands is not used after this vperm instruction then it can be
substituted by xxperm which doubles the number of available registers.

Reviewed By: stefanp

Differential Revision: https://reviews.llvm.org/D133700