[RISCV] Bugfix for 90f91683 noticed in follow up work

I went to extend this locally, and then promptly tripped across a bug which is possible with the landed patch.  The problematic case is:
vsetvli zero, 4, <some vtype>
vmv.x.s x1, v0
vsetvli a0, zero, <same type>

In this case, the naive rewrite - what I had implemented - would form:
vsetvli zero, zero, <same vtype>
vmv.x.s x1, v0

This is, amusingly, correct for the vmv.x.s, but is incorrect for the instructions which follow the sequence and probably rely on VL=VLMAX.  (The VL before the sequence is unknown, and thus doesn't have to be VLMAX.)

I plan to rework the rewrite code to be more robust here, but I wanted to directly fix the bug first.  Sorry for the lack of test; I didn't manage to reproduce this without an additional optimization change after a few minutes of trying.