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

[Luke Lau via llvm-commits]

lukel97 wrote:

> The forward version of (the VTYPE toggle subset of) this patch turned out to be really straight forward. I posted it here: #69759
> 
> I suggest we review and land that one, then rebase this to see what is left. Depending on that, we may or may not decide to move forward with this. Let's see if the complexity is still justified.

I agree with trying to move this into the forward phase, the "generalisation" I was alluding to in the previous comment was able to move this into `RISCVInsertVSETVLI::transferBefore` which didn't require any MachineInstr manipulation

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