[RISCV] Use maskedoff to decide mask policy for masked compare and vmsbf/vmsif/vmsof.

masked compare and vmsbf/vmsif/vmsof are always tail agnostic, we could
check maskedoff value to decide mask policy rather than have a addtional
policy operand.

Reviewed By: craig.topper, arcbbb

Differential Revision: https://reviews.llvm.org/D122456