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

[Nhat Nguyen via llvm-commits]

changkhothuychung wrote:

@RKSimon RkSim
What type of test should I add, should it be similar to this test ? 

https://github.com/llvm/llvm-project/blob/ffe41819e58365dfbe85a22556c0d9d284e746b9/llvm/unittests/Support/KnownBitsTest.cpp#L834-L848

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