[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:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:  #1498
             Test Case:              |
              Blocking:  8317        |
Differential Revisions:              |
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Comment (by jstolarek):

 Replying to [comment:3 dfeuer]:
 > If I hand it `12#`, it will dutifully tell me `False`. What exactly have
 I learned from that?
 That the argument you passed is not a valid tag for `True`.

 This ticket is not really about `isTrue#` or `isFalse#` - which are just
 tools a programmer might want or not want to use - but about fixing the
 heap checks and thus fixing #8317. If you feel that `isTrue#` and
 `isFalse#` don't offer you any benefit you can still use `tagToEnum#`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8326#comment:7>
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