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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@changkhothuychung If you change the KnownBits implementation back to the same as the original SelectionDAG (add/sub of ext) do the problems go away?

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