[llvm] [DAGCombiner] Preserve nneg flag from inner zext when we combine (z/s/aext (zext X)) (PR #82199)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

The changes LGTM, but is it possible to test it (or at least some parts of it)?

https://github.com/llvm/llvm-project/pull/82199