[GHC] #8555: Simplify given `Coercible` constraints

[GHC]

#8555: Simplify given `Coercible` constraints
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        Reporter:  nomeata                    |            Owner:
            Type:  feature request            |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler (Type checker)    |          Version:  7.7
      Resolution:                             |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:  #8503
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Comment (by goldfire):

 Certainly nothing blocking this at the Core level... the `Nth` coercion
 constructor would do the right thing here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8555#comment:1>
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