patch applied (ghc-6.8/ghc): FIX Trac 1662: actually check for existentials in proc patterns

[Ian Lynagh]

Wed Nov 14 03:29:30 PST 2007  simonpj at microsoft.com
  * FIX Trac 1662: actually check for existentials in proc patterns
  
  I'd fixed the bug for code that should be OK, but had forgotten to 
  make the test for code that should be rejected! 
  
  Test is arrowfail004
  

    M ./compiler/typecheck/TcArrows.lhs -1 +1
    M ./compiler/typecheck/TcPat.lhs -20 +35