Don't expose the unfolding of dictionary selectors without -O

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.