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

[Ulrich Weigand via llvm-commits]

uweigand wrote:

> LG.
> 
> @uweigand : This makes me think that someone from SystemZ might wanna add a fallback call to TargetLowering::isGuaranteedNotToBeUndefOrPoisonForTargetNode now when the function actually is doing something.

Thanks for the hint!  @JonPsson can you look into this?

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