[ARM] MVE loads and stores

This fills in the gaps for basic MVE loads and stores, allowing unaligned
access and adding far too many tests. These will become important as
narrowing/expanding and pre/post inc are added. Big endian might still not be
handled very well, because we have not yet added bitcasts (and I'm not sure how
we want it to work yet). I've included the alignment code anyway which maps
with our current patterns. We plan to return to that later.

Code written by Simon Tatham, with additional tests from Me and Mikhail Maltsev.

Differential Revision: https://reviews.llvm.org/D63838

llvm-svn: 364633