Note [Redundant constraints in instance decls]

Instance declarations are special in two ways:

* We don't report unused givens if they can give rise to improvement.
  Example (#10100):
    class Add a b ab | a b -> ab, a ab -> b
    instance Add Zero b b
    instance Add a b ab => Add (Succ a) b (Succ ab)
  The context (Add a b ab) for the instance is clearly unused in terms
  of evidence, since the dictionary has no fields.  But it is still
  needed!  With the context, a wanted constraint
     Add (Succ Zero) beta (Succ Zero)
  we will reduce to (Add Zero beta Zero), and thence we get beta := Zero.
  But without the context we won't find beta := Zero.

  This only matters in instance declarations.

* We don't report givens that are a superclass of another given. E.g.
       class Ord r => UserOfRegs r a where ...
       instance (Ord r, UserOfRegs r CmmReg) => UserOfRegs r CmmExpr where
  The (Ord r) is not redundant, even though it is a superclass of
  (UserOfRegs r CmmReg).  See Note [Recursive superclasses] in GHC.Tc.TyCl.Instance.

  Again this is specific to instance declarations.