Make when comparing old and new strictness information,
we were being a bit too generous with the old case.
When a function has (say) arity 2, we can't use strictness
info like SSS, because that only applies if the fn is
applied to 3 args.  So it's an unfair comparison.

This commit makes the old->new conversion function more
truthful in this regard, which should eliminate the
erroneously-claimed "worse" strictness results.