[llvm] [DAG] foldAddToAvg - add patterns to form avgceil(A, B) from ((A >> 1) + (B >> 1)) + ((A | B) & 1) (PR #174719)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon auto_merge_enabled https://github.com/llvm/llvm-project/pull/174719