[GHC] #8609: Clean up block allocator

[GHC]

#8609: Clean up block allocator
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        Reporter:  ezyang            |            Owner:  ezyang
            Type:  bug               |           Status:  new
        Priority:  low               |        Milestone:
       Component:  Runtime System    |          Version:  7.7
      Resolution:                    |         Keywords:  patch
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  Runtime           |       Difficulty:  Easy (less than 1
  performance bug                    |  hour)
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonmar):

 Looks good to me!  Nice catch for the duplicate `initGroup` call.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8609#comment:1>
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