[GHC] #15100: `ApplicativeDo` needlessly uses `join` too much

[GHC]

#15100: `ApplicativeDo` needlessly uses `join` too much
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        Reporter:  ekmett            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.2
  (CodeGen)                          |
      Resolution:                    |             Keywords:  ApplicativeDo
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 dfeuer):

 This is all around `stmtTreeToStmts` in `compiler/rename/RnExpr`, and
 `needJoin` seems rather relevant.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15100#comment:2>
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