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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> This is already part of #84693, but I'm fine with landing it separately first.

Eeek - sorry about that - I was just going through my TODO list and you mentioned it a few weeks back .......

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