move and generalize another instance (#1405)

move and generalize another instance (#1405)
was instance Outputable CmmGraph
type CmmGraph = LGraph Middle Last
now instance (ctx) => Outputable (LGraph m l),
in module with LGraph where it belongs
This also let us reduce the context of DebugNodes to Haskell98,
leaving that class's only extension being multi-parameter.
(also Outputable (LGraph M Last) with M = ExtendWithSpills Middle
was another redundant instance that was then removed)