UndecidableInstances not needed for equality constraints

Consider

instance (Op c, In c ~ Int) => Op (D2 c) where ...

Without -XUndecidableInstaces we get

undec.hs:21:0:
     Constraint is no smaller than the instance head
       in the constraint: In c ~ Int
     (Use -XUndecidableInstances to permit this)

But the undecidable-instance check is only needed for class constraints I think, so GHC is being over conservative. Let's fix this.

Trac metadata

Trac field	Value
Version	6.12.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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