[llvm] [SLP]Improve minbitwidth analysis. (PR #78976)

[via llvm-commits]

dyung wrote:

Definitely failing on my GCC based builders:
- https://lab.llvm.org/buildbot/#/builders/139/builds/60749
- https://lab.llvm.org/buildbot/#/builders/247/builds/14934
- https://lab.llvm.org/buildbot/#/builders/217/builds/37166
- https://lab.llvm.org/buildbot/#/builders/243/builds/21356

https://github.com/llvm/llvm-project/pull/78976