Commits on May 20, 2024

1. [DivRemPairs] Do not freeze poisons that can't be undef …

   Per comments in DivRemPairs, the rewrite from
   ```llvm
       %div = div %X, %Y
       %rem = rem %X, %Y
   ```
   to
   ```llvm
       %div = div %X, %Y
       %.mul = mul %div, %Y
       %rem = sub %X, %mul
   ```
   is unsound when %X or %Y are undef.
   
   However, it is known to be sound if %X or %Y are poison but can't be
   undef, since both the pre- and post-rewrite %rem are `poison`.
   
   Additionally, proofs: https://alive2.llvm.org/ce/z/xtNQ8j
   
   A comment in the pass listed a TODO for changing a usage of
   isGuaranteedNotToBeUndefOrPoison() in the pass to something that only
   detects undef. Such a function has been implemented since the time
   that TODO was written, but has not been used.
   
   Therefore, this commit updates DivRemPairs to use
   isGuaranteedNotToBeUndef() instead.

   

   krzysz00 committed May 20, 2024
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