Optimisation: the update code only needs to check whether the closure
being updated is a BLACKHOLE_BQ if it has been blackholed; which is
only the case after lazy blackholing has taken place.

So to avoid this unnecessary cost, we have two kinds of update frame:
one which checks for BLACKHOLE_BQ and one that doesn't, and the lazy
blackholing algorithm converts the latter kind into the former as it
replaces thunks with blackholes.