[ARM] MVE reverse shuffles.

The vectorizer can sometimes make reverse shuffles from indices that
count down. In MVE, we don't have a 128bit rev instruction, but we can
select this to a VREV64 with some lane movs to swap the two halfs.

Ideally this would use VMOVD's, but only gets as far as VMOVS's at the
moment.

Differential Revision: https://reviews.llvm.org/D69510