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

[Philip Reames via llvm-commits]

preames wrote:

The forward version of this patch turned out to be really straight forward.  I posted it here: https://github.com/llvm/llvm-project/pull/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.  

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