[RISCV][VLOpt] Consolidate EMUL=SEW/EEW*LMUL logic [NFC]

All but one of the cases in tree today have EMUL=SEW/EEW*LMUL.  Repeating
this each time is verbose and introduces oppurtunity for error.  (For instance,
the comment associated with vwmul.vv was out of sync with the code for same.)

Introduce getOperandLog2EEW and move most complexity to it.  Then introduce
getOperandInfo as a wrapper around previous, and special case the one case
which requires it.

Author: preames