[GHC] #7206: Implement cheap build

[GHC]

#7206: Implement cheap build
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        Reporter:  simonpj           |            Owner:  simonpj
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  _|_
       Component:  Compiler          |          Version:  7.4.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonpj):

 If we do this, don't forget to use `cheapBuild` for `repeat`.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7206#comment:6>
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