Refactor the code that prevents recursion among Coercible constraints
The main description is in Note [Preventing recursive dictionaries] in TcRnTypes, which applies only to Coercible dictionaries. But it was a bit of a mess: - It wasn't applied consistently - It was being applied to non-Coercible dictionaries in some places This patch tidies it up. This hack will largely go away when Richard starts treating Coercible constraints more like equalities than like dictionaries.
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