Sanjay Patel f0faea5714 [InstSimplify] fold exact divide to poison if it is known to not divide evenly ...

This is related to the discussion in D140665. I was looking over the demanded
bits implementation in IR and noticed that we just bail out of a potential
fold if a udiv is exact:
82be8a1d2b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp (L799)

Also, see tests added with 7f0c11509e.

Then, I saw that we could lose a fold to poison if we zap the exact with that
transform, so this patch tries to catch that as a preliminary step.

Alive2 proofs:
https://alive2.llvm.org/ce/z/zCjKM7
https://alive2.llvm.org/ce/z/-tz_RK (trailing zeros must be "less-than")
https://alive2.llvm.org/ce/z/c9CMsJ (general proof and specific example)

Differential Revision: https://reviews.llvm.org/D140733

2022-12-29 10:26:50 -05:00

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