[PowerPC] Don't combine SCALAR_TO_VECTOR without VSX

Most of the patterns for PPCISD::SCALAR_TO_VECTOR_PERMUTED require
VSX. So don't emit them if the subtarget doesn't have VSX.
This resolves the issue reported on
https://reviews.llvm.org/rG1fed131660b2c5d3ea7007e273a7a5da80699445