[RISCV][VLOpt] Consolidate EMUL=SEW/EEW*LMUL logic [NFC] (llvm#122021)

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.

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Co-authored-by: Luke Lau <[email protected]>

Author: preames