[GHC] #13823: Use NonEmpty lists in more places in the GHC API

[GHC]

#13823: Use NonEmpty lists in more places in the GHC API
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        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8782             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Well, I'm looking to avoid partially retrieving the representative of a
 list of duplicates, by hook or by crook. Do you have an idea in mind for a
 better `ListSetOps` replacement?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13823#comment:3>
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