This covers the case where we insert a scalar into lane 0 of a vector after performing the slideup. This was triggered by offline discussion about D151736, and is an alternative to that approach. I think this is cleaner and a potentially more general.
A couple things we have to be careful of:
- The type sizes need to match. We implicitly truncate the scalar, and the shift is in SEW units.
- We need to avoid VL=0 and a mask with lane-0 disabled. The former is just tricky to reason about, the later differs in behavior between the two forms.