[RISCV] Split usage of VPseudoUnaryNoMask with GPR destination

These instructions don't have a passthrough operand or any of the policy behaviors, while are the other ones do. Split them out into their own class to make this separation clear, and rename the mask variant to match. (We'd already done the same for the mask variant.)

Differential Revision: https://reviews.llvm.org/D153596