[llvm] [DAG] foldShiftToAvg - recognize sub(x, xor(y, -1)) >> 1 as avgceil[su] (PR #182616)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@laurenmchin don't forget to keep pinging - otherwise the pr tends to disappear down my notification list

https://github.com/llvm/llvm-project/pull/182616