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

[Nhat Nguyen via llvm-commits]

changkhothuychung wrote:

Thanks everyon! looks like I got 2 approves already, I will merge this PR shortly if there is no other concern.

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