[GHC] #9623: Use Data.List.dropWhileEnd

[GHC]

#9623: Use Data.List.dropWhileEnd
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              Reporter:  dfeuer      |            Owner:
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  libraries   |          Version:  7.8.3
  (other)                            |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Easy (less than 1
  Unknown/Multiple                   |  hour)
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by refold):

 Replying to [comment:8 dfeuer]:
 > However, the above `dropWhileEndLE` appears to be faster than the double
 reverse in all cases. So I'll add it to the utilities module and use that
 instead.

 Since `dropWhileEndLE` is both faster and has less surprising semantics
 (spine-strict instead of element-strict), I think that the standard
 library should be changed to use this version. And we should also add
 `takeWhileEnd` while we're at it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9623#comment:9>
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