[DivRemPairs] Do not freeze poisons that can't be undef (#92627)

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.