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

[Luke Lau via llvm-commits]

lukel97 wrote:

Marking as a draft as there may be a more general way of doing this by adjusting the the SEW/LMUL ratio and converting to vsetvli zero,zero separately

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