[RISCV] Define vmclr.m/vmset.m intrinsics.

Define vmclr.m/vmset.m intrinsics and lower to vmxor.mm/vmxnor.mm.

Ideally all rvv pseudo instructions could be implemented in C header,
but those two instructions don't take an input, codegen can not guarantee
that the source register becomes the same as the destination.

We expand pseduo-v-inst into corresponding v-inst in
RISCVExpandPseudoInsts pass.

Reviewed By: craig.topper, frasercrmck

Differential Revision: https://reviews.llvm.org/D93849