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.
I define two pseduo insts, one would expand to vsetvli + pseduo-v-inst,
then pseudo-v-inst would expand to corresponding v-inst.
nit: the class is called RISCVNullaryIntrinsic producing llvm_anyvector_ty but says it outputs "mask type". Is that too specific? The other intrinsic classes which output "mask type" are called e.g. RISCVMaskXXX. Should this be named e.g. RISCVMaskNullary(Intrinsic)? I don't think we expect other nullaries, do we?