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

[Nhat Nguyen via llvm-commits]

changkhothuychung wrote:

@RKSimon RK
it looks like there is a failed build in later stages after the PR was merged - https://lab.llvm.org/buildbot/#/builders/185/builds/6813

Should we revert the changes? 

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