[PowerPC] Fix PPCVSXSwapRemoval pass to include MTVSCR and MFVSCR as not swappable.

This patch adds the instructions `MTVSCR` and `MFVSCR` as not swappable to the
PPCVSXSwapRemoval pass because they are not lane-insensitive. This will prevent
the compiler from optimizing out required swaps when using `lxvd2x` and
`stxvd2x`.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D128062