[llvm] [RISCV] Increase default vectorizer LMUL to 8 (PR #222213)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> > Can we assess the impact on register pressure by the statistics?
> 
> Yes, we can. I've also seen that this increases register spilling in certain benchmarks. I'll investigate the root cause and work on a fix in the Loop Vectorizer.

I did some investigations in https://github.com/llvm/llvm-project/pull/216005, please feel free to take them if they are helpful.

https://github.com/llvm/llvm-project/pull/222213