With AVX the performance for aligned vector move and unaligned vector move on X86
are the same if the address is aligned. In this case we prefer to use unaligned
move because it can avoid some run time exceptions.
"-muse-unaligned-vector-move" and "-mno-use-unaligned-vector-move" are added to
enable this preference. This transform is disabled as default.
This patch is a replacement of D88396.