[llvm] [DAG] SelectionDAG::canCreateUndefOrPoison - AVGFLOORU/AVGCEILU don't create undef/poison (PR #157056)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> It is intuitively obvious to me that AVGFLOORU/AVGCEILU/AVGFLOORS/AVGCEILS have a well defined result for all (non-undef non-poison) inputs, so I think this is fine even without Alive2 proofs.

Are you happy if I add AVGFLOORS/AVGCEILS handling top this patch as well?

https://github.com/llvm/llvm-project/pull/157056