[llvm] [SelectionDAG] implement computeKnownBits for add AVG* instructions (PR #86754)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> Will we merge it, or should I add anything?

Sorry - I've been away for the Easter holidays - I've committed this now, so we just need to complete #76644 to properly close #53622

https://github.com/llvm/llvm-project/pull/86754