[llvm] [DAG] fold avgu(zext(x), zext(y)) -> zext(avgu(x, y)) (PR #95134)

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c8ef wrote:

I would like to continue working on the signed case. Do we need to explicitly use `SelectionDAG::getFreeze` for the signed extended source value to match the alive test?

https://github.com/llvm/llvm-project/pull/95134