Note [Equality superclasses]

Suppose we have
  class (a ~ [b]) => C a b

Remember from Note [The equality types story] in GHC.Builtin.Types.Prim, that
  * (a ~~ b) is a superclass of (a ~ b)
  * (a ~# b) is a superclass of (a ~~ b)

So when closeWrtFunDeps expands superclasses we'll get a (a ~# [b]) superclass.
But that's an EqPred not a ClassPred, and we jolly well do want to
account for the mutual functional dependencies implied by (t1 ~# t2).
Hence the EqPred handling in closeWrtFunDeps.  See #10778.