[llvm] [DAG] SelectionDAG::canCreateUndefOrPoison - Mark AVGFLOORS and AVGCEILS as safe (PR #148191)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

What could be done is to fold based on the knownbits/signbits result of a avgu/avgs node - if the freeze is moved then the knownbits/signbits calculation is exposed and can allow the fold to occur.

https://github.com/llvm/llvm-project/pull/148191