[llvm] [DAG] fold `avgs(sext(x), sext(y))` -> `sext(avgs(x, y))` (PR #95365)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Please can you rebase + regenerate the tests now that #95415 has landed

https://github.com/llvm/llvm-project/pull/95365