[llvm] [InstCombine] Canonicalize more saturated add variants (PR #100008)

[via llvm-commits]

AtariDreams wrote:

> Please replace tests using something like uge C with ugt C-1 so the input is canonical. And then remove all the tests that become duplicates as a result.
> 
> Your alive2 proofs also include some preconditions -- some cases check that C!=0. But it does not look like your implementation contains those checks?

It does now!

https://github.com/llvm/llvm-project/pull/100008