[RISCV] Add VFCVT pseudos with no mask

When emitting a vfcvt with a rounding mode, we end up generating an unnecessary
vmset because the only rounding mode pseudos have a mask operand. This patch
adds a pseudo without a mask, and marks the masked variant with the
MaskedPseudo class so the doPeepholeMergeVMV optimisation knows to remove the
redundant vmset.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D154266