[GHC] #8326: Place heap checks common in case alternatives before the case

[GHC]

#8326: Place heap checks common in case alternatives before the case
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        Reporter:  jstolarek         |                   Owner:
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.7
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:  8317
 Related Tickets:  #1498             |  Differential Revisions:  Phab:D343
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Comment (by bgamari):

 Actually, now that I look a bit more carefully `StgCmmMonad.codeOnly` may
 be exactly what is needed here. I'll need to think a bit more to make sure
 this breaks all of the potential cycles, but I am optimistic.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8326#comment:31>
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