[RISCV] Use mask agnostic policy for isel patterns where the merge operand is IMPLICIT_DEF.

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.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D135386