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.
8b22bb8a62