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

[Ralf Jung via llvm-commits]

RalfJung wrote:

For Rust, in the future we'd like to have poison semantics for our values, and then we cannot use `nopoison`. More precisely, today we could use `nopoison`, but it would be a shame if LLVM wouldn't let its frontends have poison semantics for values without codegen regressions. Alternatives exist, as mentioned in [my comment](https://discourse.llvm.org/t/rfc-add-nopoison-attribute-metadata/79833/12?u=ralfjung).

Today, Rust doesn't yet have values with poison semantics (it is regularly requested but we are blocked on lack of support in LLVM). So today, `nopoison` could be added in Rust. So if this is some sort of transition strategy that's perfectly fine, but it is not a great long-term solution.

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