Make simplifyInfer generalise only over simple class constraints

So we never infer
   f :: Eq (Tree a) => blah
when there isn't an instance for Eq (Tree a).

This fixes Trac #6022.

It does represent a change in behaviour: certain (bizarre) programs
will be rejected that were previously accepted. Specifically, if you
have

   module A where
   f x = ...somethign needing (C T)...

   moudule B where
   import A
   instance C T
   test = f True

Here the (C T) instance is provided "later".  But this is wierd; it
would be better to give a type signature for f
   f :: C T => Bool -> Bool
and then you'd be fine.