[llvm] [LLVM][ConstantFold] Undefined values are not constant (PR #130713)

[Kees Cook via llvm-commits]

kees wrote:

> [Undef](https://llvm.org/docs/LangRef.html#undefined-values) is now deprecated and should only be used in the rare cases where no replacement is possible.

AIUI, this test really does want to use `undef` and not `poison`.

https://github.com/llvm/llvm-project/pull/130713