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 will, 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.