[PowerPC] Don't generate ST_VSR_SCAL_INT if power8-vector is disabled

Summary:
In https://bugs.llvm.org/show_bug.cgi?id=45297, it fails selecting
instructions for `PPCISD::ST_VSR_SCAL_INT`. The reason it generate the
`PPCISD::ST_VSR_SCAL_INT` with `-power8-vector` in IR is PPC's
combiner checks `hasP8Altivec` rather than `hasP8Vector`. This patch
should resolve PR45297.

Differential Revision: https://reviews.llvm.org/D76773