[RISCV] Masked compares should use a tail agnostic policy.

Writes of a mask result are always tail agnostic.

Unfortunately, this seems to have made codegen worse. I can only
think this must be because the vsetvli was acting as some sort
of barrier that prevented some code movement in the scheduler.

Reviewed By: arcbbb

Differential Revision: https://reviews.llvm.org/D103331