[GHC] #11158: Combine exprIsTrivial and cpe_ExprIsTrivial

[GHC]

#11158: Combine exprIsTrivial and cpe_ExprIsTrivial
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        Reporter:  simonpj           |                Owner:  bgamari
            Type:  task              |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       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):

 When quickly reviewing Phab:D1656 in response to some validation issues I
 noticed that these `exprIsTrivial` variants differ slightly in their
 treatment of literals.

 While `CorePrep`'s `cpe_ExprIsTrivial` always considers `Lits` to be
 trivial, `CoreUtils`'s variant relies on `Literal.litIsTrivial` in this
 case. `litIsTrivial` deems all literals except `MachStr` and `LitIntegers`
 to be trivial.

 This effectively means that `cpeArg` will no longer consider these
 literals to be trivial, resulting in eta expansion of some expressions
 containing them which are currently left untouched. This is evidently
 problematic.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11158#comment:5>
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