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

[GHC]

#8555: Simplify given `Coercible` constraints
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        Reporter:  nomeata           |                   Owner:
            Type:  feature request   |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.7
  checker)                           |                Keywords:
      Resolution:  fixed             |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |  typecheck/should_compile/T8555
      Blocked By:                    |                Blocking:
 Related Tickets:  #8503 #8555       |  Differential Revisions:
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Changes (by simonpj):

 * testcase:   => typecheck/should_compile/T8555


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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8555#comment:10>
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