[GHC] #2790: Use -fregs-graph by default

[GHC]

#2790: Use -fregs-graph by default
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    Reporter:  igloo             |       Owner:                  
        Type:  task              |      Status:  new             
    Priority:  high              |   Milestone:  6.12 branch     
   Component:  Compiler          |     Version:  6.8.3           
    Severity:  normal            |    Keywords:                  
  Difficulty:  Unknown           |    Testcase:                  
Architecture:  Unknown/Multiple  |          Os:  Unknown/Multiple
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 Check performance of `-fregs-graph` vs the default (linear0scan
 allocator). Assuming that all is well, make `-fregs-graph` the new
 default, and add a `-fregs-linear` flag for the current behaviour.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2790>
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