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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

I think this is a good direction, considering what additional work I have planned. I will give a more through review tomorrow.

https://github.com/llvm/llvm-project/pull/122021