[GHC] #8894: Clean up `Coercible` plumbing

[GHC]

#8894: Clean up `Coercible` plumbing
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        Reporter:  nomeata           |            Owner:  nomeata
            Type:  task              |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.9
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #8904
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Comment (by Joachim Breitner <mail@…>):

 In [changeset:"7511d5bf708f38bbdf5733f42dc8a025c76cc684/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="7511d5bf708f38bbdf5733f42dc8a025c76cc684"
 Fix validation issue due to Coercible move (#8894)
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8894#comment:11>
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