[PowerPC] Enhance the selection(ISD::VSELECT) of vector type

To make ISD::VSELECT available(legal) so long as there are altivec instruction, otherwise it's default behavior is expanding,
which is legalized at type-legalization phase. Use xxsel to match vselect if vsx is open, or use vsel.


Differential Revision: https://reviews.llvm.org/D49531

llvm-svn: 346824