[llvm] [DemandedBits] Support non-constant shift amounts (PR #148880)

[Panagiotis Karouzakis via llvm-commits]

karouzakisp wrote:

> Please provide the alive2 proof. See also my previous comment [#148853 (review)](https://github.com/llvm/llvm-project/pull/148853#pullrequestreview-3021204187)

@dtcxzyw Here are the alive2 proofs -->
https://alive2.llvm.org/ce/z/SxgY_5

Please note that since my transformation contains a loop and the Alive syntax doesn't permit loops, I added various ranges.

Please let me know if it's okay. 

@artagnon, Please let me know what you think. 

https://github.com/llvm/llvm-project/pull/148880