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

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

> This issue is still open. To give a specific example, you are currently miscompiling this test case: https://alive2.llvm.org/ce/z/66TJWX

Can you please also add a test for this?

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