[llvm] [InstCombine] Fold `[l|a]shr iN (X-1)&~X, N-1 -> [z|s]ext(X==0)` (PR #107259)

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goldsteinn wrote:

Also think a better place for this would be `simplifyDemandedBits`. There are a lot of patterns in the bits based on `X` and `C`(1) in this case.

https://github.com/llvm/llvm-project/pull/107259