[llvm] [SLP]Improve minbitwidth analysis. (PR #84536)

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

Looks like 7f2167868d8c1cedd3915883412b9c787a2f01db broke aarch64 bootstrap.

https://lab.llvm.org/buildbot/#/builders/176
(and our private builder)

https://github.com/llvm/llvm-project/pull/84536