This takes the approach of using the loop based formation for scalable vectors only. We could potentially use the loop form for fixed vectors only, but we'd loose the unroll and specialize on constant vector logic which is already present. I don't have a strong opinion on whether the existing logic is worthwhile, I kept it mostly to minimize test churn.
Worth noting is that there is a better lowering available. The plain vector lowering appears to check only the first and last byte. By analogy, we should be able to check only the first active and last active byte in the masked op. This is a more invasive change to asan, and I decided simply supporting scalable vectors at all was a better starting place.