[llvm] [llvm] Add KnownBits implementations for avgFloor and avgCeil (PR #86445)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

#86754 has now landed so please can you update the SelectionDAG::computeKnownBits AVG cases to use KnowBits implementations

https://github.com/llvm/llvm-project/pull/86445