[GHC] #9127: Don't warn about pattern-bindings of the form `let !_ = rhs`

[GHC]

#9127: Don't warn about pattern-bindings of the form `let !_ = rhs`
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        Reporter:  refold            |            Owner:  gzayas
            Type:  bug               |           Status:  patch
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      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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Comment (by nomeata):

 Reviewed, validated and pushed, thanks Guido for your first contribution
 to GHC!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9127#comment:9>
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