[PowerPC] Don't crash on larger splats achieved through 1-byte splats

We've implemented a 1-byte splat using XXSPLTISB on P9. However, LLVM will
produce a 1-byte splat even for wider element BUILD_VECTOR nodes. This patch
prevents crashing in that situation.

Differential Revision: https://reviews.llvm.org/D35650

llvm-svn: 310358