Most vector instructions define tail elements at those being past VL. (vmv.s.f and vmv.s.x are defined differently.) When VL==VLMAX, there are no tail elements, so tail agnostic and tail undisturbed are equivalent. Canonicalize to TA as that gives us more flexibility.
Honestly, this should really be done earlier, but then we'd need to have policy flags on all the psuedos. I started down that path, and then decided this was radically simpler.
Can fix the spelling of "sentinel" while you're here?