[RISCV] Minor improvements to RISCVInstrInfoXSf.td. NFC (#76424)

Use an explicit list of LMULInfos instead of indexing part of other
lists. Use wvrclass field to double LMUL instead of using two lists.

Use range instead of list in another spot. I wish I could use a list of
LMULInfo here but there's no way to quadruple an LMUL yet.