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

[via llvm-commits]

goldsteinn wrote:

> > Any luck with fixing the unit test failures?
> 
> no not yet unfortunately, it should be related to the lines in the file SelectionDAG.cpp I highlighted above, be cause the failed test links to those lines, but currently I still struggle figuring out why it fails.

Just some tips for reproducing locally.
For the known bits unittests you can do:
```
$> ninja UnitTests && unittests/Support/./SupportTests --gtest_filter=KnownBitsTest*
```

For the DAGCombiner tests you can do:
```
$> ninja && ./bin/llvm-lit ../llvm/test/CodeGen/
```

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