Add MVE VMULH/VRMULH intrinsics and unit tests.
Looks good to me, with a couple of minor nitpicks (that I'm not sure matter or not).
Any strong reason to switch these around? From what I can tell most of the patterns are (suffix, unsigned, size) in that order. Although that doesn't seem to be universal. Might as well try to keep it consistent, if you do not have a reason to swap them.