[PowerPC] Use vector types for memcpy and friends (sometimes)

When using Altivec, we can use vector loads and stores for aligned memcpy and
friends. Starting with the P7 and VXS, we have reasonable unaligned vector
stores. Starting with the P8, we have fast unaligned loads too.

For QPX, we use vector loads are stores, but only for aligned memory accesses.

llvm-svn: 230788