Don't expose the unfolding of dictionary selectors without -O
When compiling without -O we were getting code like this
f x = case GHC.Base.$f20 of
:DEq eq neq -> eq x x
But because of the -O the $f20 dictionary is not available, so exposing
the dictionary selector was useless. Yet it makes the code bigger!
Better to get
f x = GHC.Base.== GHC.Bsae.$f20 x x
This patch suppresses the implicit unfolding for dictionary selectors
when compiling without -O. We could do the same for other implicit
Ids, but this will do for now.
There should be no effect when compiling with -O. Programs should
be smaller without -O and may run a tiny bit slower.