[GHC] #8719: clarify prefetch release notes + remove some deadcode

[GHC]

#8719: clarify prefetch release notes + remove some deadcode
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        Reporter:  carter            |            Owner:
            Type:  bug               |           Status:  merge
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.8.1-rc1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Changes (by thoughtpolice):

 * status:  patch => merge


Comment:

 Merged in HEAD. The 'dead code' is not actually dead code, because by
 removing it `./validate` gets angry due to incomplete pattern matches.

 I've cherry picked the documentation patch.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8719#comment:4>
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