[llvm] [KnownBits] Make `avg{Ceil,Floor}S` and `{s,u}{add,sub}_sat` optimal (PR #110329)

[Jay Foad via llvm-commits]

jayfoad wrote:

There's a lot to understand here. Could you split it into two patches, for avg and add/sub sat? Or are they inextricably intertwined?

https://github.com/llvm/llvm-project/pull/110329