[GHC] #12915: cmmImplementSwitchPlans creates duplicate blocks

[GHC]

#12915: cmmImplementSwitchPlans creates duplicate blocks
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        Reporter:  alexbiehl         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by hsyl20):

 Could we allow this additional pass with an optimization flag? It would be
 a good candidate for a `-O2` (“Apply every non-dangerous optimisation,
 even if it means significantly longer compile times.” so you get what you
 ask for) or a potential `-Os`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12915#comment:8>
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