I tend to think we should ignore the policy bit in vsetvli insertion
if the tied operand is IMPLICIT_DEF. But that raises questions about
what the policy operand on RVV intrinsics means if you also pass
vundefined().
This change at least fixes some cases. I'll post a separate patch
for vsetvli insertion for discussion.
Slide down has special wording around the handling of the mask. Could you move this to it's own patch? I think this needs careful consideration on it's own.
(Same for slide up)