[GHC] #15019: Fix performance regressions from #14737

[GHC]

#15019: Fix performance regressions from #14737
-------------------------------------+-------------------------------------
           Reporter:  tdammers       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #14737
Differential Rev(s):  D4568          |         Wiki Page:
-------------------------------------+-------------------------------------
 #14737, while drastically improving performance on some edge cases,
 introduces performance regressions in 3 test cases:

 perf/compiler/parsing001.run
 perf/compiler/T9020.run
 perf/compiler/T12707.run

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15019>
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