[llvm] [RISCV] Adjust LMUL if not used to avoid VL toggle (PR #69259)

[Luke Lau via llvm-commits]

lukel97 wrote:

This remediates the VL toggles that would be introduced if we decided to remove the LMUL variants for VMV_S_X pseudos as mentioned here: https://github.com/llvm/llvm-project/pull/66659#issuecomment-1753450731

https://github.com/llvm/llvm-project/pull/69259