[PowerPC] Exploit single instruction load-and-splat for word and doubleword

We currently produce a load, followed by (possibly a move for integers and) a
splat as separate instructions. VSX has always had a splatting load for
doublewords, but as of Power9, we have it for words as well. This patch just
exploits these instructions.

Differential revision: https://reviews.llvm.org/D63624

llvm-svn: 372139