patch applied (ghc): Improve exprIsCheap

[Simon Peyton Jones]

Mon Aug 14 09:50:43 PDT 2006  simonpj at microsoft.com
  * Improve exprIsCheap
  
  exprIsCheap is meant to return True iff it's ok to push the expression
  inside a lambda.  But the previous version would return True of a nested
  construtor application like (1:2:3:[]), which isn't right.
  
  This patch re-factors the code somewhat, and fixes the bug.
  

    M ./compiler/coreSyn/CoreUtils.lhs -39 +47