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

[Jay Foad via llvm-commits]

jayfoad wrote:

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

Oh, I thought this implementation was supposed to be optimal, but maybe I got that wrong.

Patch LGTM anyway.

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