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

[Nhat Nguyen via llvm-commits]

changkhothuychung wrote:

@jayfoad f
looks like the BinaryExhaustive test fails when I remove the bool flag to enable Optimal check? 

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