[GHC] #11306: Do not generate warning in `do` when result is of type `Void`.

[GHC]

#11306: Do not generate warning in `do` when result is of type `Void`.
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        Reporter:  diatchki          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      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 rwbarton):

 I'm not a fan of this warning in the first place, but if anything, I would
 think it is most important to warn when the discarded value is of type
 `Void`, since that typically indicates that the following code is
 unreachable...

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11306#comment:1>
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