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

[GHC]

#8555: Simplify given `Coercible` constraints
----------------------------------------------+----------------------------
        Reporter:  nomeata                    |            Owner:  archblob
            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
                                              |  #8555
----------------------------------------------+----------------------------

Comment (by nomeata):

 Nobody is working on it, so if you feel like it, please give it a shot.
 (But also beware that in the last months, nobody was missing the feature,
 so it may have a limited impact – don’t let that lessen your motivation!)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8555#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler