[llvm] [polly] [InstSimplify] Fix incorrect poison propagation when folding phi (PR #96631)

[Nikita Popov via llvm-commits]

nikic wrote:

RFC for nopoison: https://discourse.llvm.org/t/rfc-add-nopoison-attribute-metadata/79833
The select -> freeze fold: https://github.com/llvm/llvm-project/pull/96773

On a somewhat tangential note, I'm also wondering whether it would be worthwhile to add some flags on `freeze` so you can have `freeze %x` (freezes undef and poison -- used e.g. for despeculation), `freeze undef %x` (freezes undef only -- used for multi-use) and `freeze poison %x` (freezes poison only -- used e.g. for the select fold).

https://github.com/llvm/llvm-project/pull/96631