[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.
Use xxsel to match vselect if vsx is open, or use vsel.

In order to do not write many patterns in td file, promote (for vector it's
bitcast) all other type into v4i32 and only pattern match vselect of v4i32 into
vsel or xxsel.

Patch by wuzish
Differential revision: https://reviews.llvm.org/D49531

llvm-svn: 339779