[llvm] [DAG] isGuaranteedNotToBeUndefOrPoisonForTargetNode - add fallback implementation (PR #86125)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

This is already part of https://github.com/llvm/llvm-project/pull/84693, but I'm fine with landing it separately first.

https://github.com/llvm/llvm-project/pull/86125