[GHC] #11160: New exhaustiveness checker breaks ghcirun004

[GHC]

#11160: New exhaustiveness checker breaks ghcirun004
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 The cost here comes from `pruneVSABound`, in particular the `Singleton`
 case in `go`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11160#comment:2>
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