[llvm] [SLP]Improve final minbitwidth analysis attempt. (PR #87786)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

[c.ll.txt](https://github.com/llvm/llvm-project/files/15017690/c.ll.txt)
is the full pre-SLP-vectorizer IR

https://github.com/llvm/llvm-project/pull/87786