Drop special provision for opaque types in computeDenot
Drop special provision for opaque types in computeDenot

It turns out we needed that only because of the hole that's now
plugged in computeAsSeenFrom

lampepfl/dotty,som-snytt/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty,lampepfl/dotty,som-snytt/dotty,som-snytt/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty