[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:  jstolarek
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.7
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:  #1498
             Test Case:              |
              Blocking:  8317        |
Differential Revisions:  Phab:D343   |
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Comment (by jstolarek):

 BTW. I think it would be a Good Thing to make `FCode` monad an instance of
 `MonadFix` and replace `StgCmmMonad.fixC` with `mfix`. I think this would
 make code easier to read. People know what `mfix` is, while implementation
 of `fixC` might not be immediately obvious.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8326#comment:18>
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