Fix an ASSERT failure with profiling

Fix an ASSERT failure with profiling

The problem arose with this kind of thing

   x = (,) (scc "blah" Nothing)

Then 'x' is marked NoCafRefs by CoreTidy, becuase it has
arity 1, and doesn't mention any caffy things.

That in turns means that CorePrep must not float out the
sat binding to give

  sat = scc "blah" Nothing
  x = (,) sat

Rather we must generate

  x = \eta. let sat = scc "blah" Nothing
            in (,) sat eta

URGH! This Caf stuff is such a mess.