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

[Nikita Popov via llvm-commits]

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nikic wrote:

Per the alive proof in https://alive2.llvm.org/ce/z/pY63fh, you need to perform this check on C2, not on Sum. And if you do, then you don't need the additional check below.

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