[GHC] #11158: Combine exprIsTrivial and cpe_ExprIsTrivial

[GHC]

#11158: Combine exprIsTrivial and cpe_ExprIsTrivial
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        Reporter:  simonpj           |                Owner:
            Type:  task              |               Status:  infoneeded
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1656
       Wiki Page:                    |
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Comment (by bgamari):

 One thing I'm a bit confused by is the comment on
 `CoreToStg.FreeVarsInfo`,
 > INVARIANT: Any ImportBound Ids are HaveCafRef Ids Imported Ids without
 CAF refs are simply not put in the FreeVarsInfo for an expression.

 Why is this limited to import-bound `Id`s? I would have thought that any
 non-thunk `Id` which has NoCafRefs would be safe to omit from the SRT,
 regardless of where it is defined. I must be missing something here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11158#comment:29>
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