[llvm] [SLP]Improve minbitwidth analysis. (PR #84334)

[James Y Knight via llvm-commits]

jyknight wrote:

This PR was merged, without
1. any information on what the changes were since it was reverted, or
2. being reviewed, or
3. fixing -- or even acknowledging -- the xxhash miscompile I reported on the previous PR.

So, I'm going to re-revert this. I can work to get a standalone reproduction example for the xxhash issue if you need.

https://github.com/llvm/llvm-project/pull/84334