These can generally be emitted using an ext instruction or mov from the
high half. The half half extracts can be free depending on the users,
but that is not handled here, just the basic costs. It originally
included all subvector extracts, but that was toned-down to just
half-vector extracts to try and help the mid end not breakup high/low
extracts without having the SLP vectorizer create a mess using other
shuffles.
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