Cache the result of countOccupied(gen->large_objects) as gen->n_large_words (#7257)

The program in #7257 was spending 90% of its time counting the live
data in gen->large_objects.  We already avoid doing this for small
objects, but in this example the old generation was full of large
objects (actually pinned ByteStrings).

MERGED from commit 016fd74d