[llvm] Matched some basic ISD::AVGFLOORU patterns (PR #84903)

[Shourya Goel via llvm-commits]

Sh0g0-1758 wrote:

@RKSimon, I did some fiddling around with the codebase and I now have the correct implementation with the tests passing. 

https://github.com/llvm/llvm-project/pull/84903