Move eta-reduced coaxiom compatibility handling quirks into FamInstEnv.
The quirk caused an issue where GHC concluded that 'D' is possibly
unifiable with 'D a' (the two types could have the same kind if D is a
Ensure T9371 stays fixed.
Reviewers: goldfire, bgamari
Reviewed By: goldfire
Subscribers: RyanGlScott, rwbarton, carter
GHC Trac Issues: Trac #15704
Differential Revision: https://phabricator.haskell.org/D5206