[llvm] [InstCombine] Fix saturated add canonicalization (PR #97973)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

This PR has gone from handling 2 cases to 4 cases, but it doesn't seem like there is test coverage for all of them? (Please pre-commit the tests so it's visible that *all* the new code paths are actually needed.)

https://github.com/llvm/llvm-project/pull/97973