[RISCV] Fix incorrect optimization for masked vmsgeu.vi with 0 immediate.

vmsgeu.vi with 0 is always true, but in the masked with mask undisturbed
policy, we still need to keep inactive elelemt which come from maskedoff.

We could return mask directly if it's mask agnostic policy in the future.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D121080