[GHC] #15731: Add sortOn/coerce rule

[GHC]

#15731: Add sortOn/coerce rule
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Core Libraries    |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 `sortOn` uses decorate-sort-undecorate, which is only efficient if the
 passed function is sufficiently expensive.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15731#comment:2>
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