[llvm] [InstCombine] Add optimization to combine adds through zext nneg, and add testcase (PR #157723)

[Nikita Popov via llvm-commits]

================

----------------
nikic wrote:

If you use early exits like this, the transform should be moved into a separate function. Otherwise there is a high risk of accidentally skipping transforms that get added after this point.

https://github.com/llvm/llvm-project/pull/157723