Eagerly simplify inherently-coherent instances

https://downloads.haskell.org/~ghc/8.0.1/docs/html/libraries/ghc-8.0.1/src/TysPrim.html explains that ~~ and ~ are "inherently coherent", so GHC can reduce them immediately to their constraints. It seems to me that certain user-written classes can also be seen to be inherently coherent in this fashion. Specifically, given

class constraintC => C a1 a2 ...
instance constraintI => C a1 a2 ...

where a1, a2, etc., are type variables, C is inherently coherent if both of the following hold:

1. C has no methods whatsoever.
2. constraintC entails constraintI (so the constraints are effectively identical).

I believe in these cases GHC can and probably should reduce C a1 a2 immediately to constraintC. Condition (2) is more general that we really need; in realistic cases, the constraints will be identical up to alpha renaming because most people don't write obfuscated code.

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Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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