[llvm] [ConstraintElim] Fix poison check before adding intrinsic facts (PR #136291)

[Iris Shi via llvm-commits]

el-ev wrote:

### Merge activity

* **Apr 30, 9:40 AM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/136291).


https://github.com/llvm/llvm-project/pull/136291