[RISCV] Prune unnecessary vector pseudo instructions. NFC

For .vf instructions, we don't need MF8 pseudos for f16. We don't
need MF8 or MF4 pseudos for f32. Or MF8, MF4, MF2 for f64.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D116437