patch applied (ghc): check for unreachable code only with -DDEBUG

[nr at eecs.harvard.edu]

Tue Sep 11 07:24:10 PDT 2007  Norman Ramsey <nr at eecs.harvard.edu>
  * check for unreachable code only with -DDEBUG

    M ./compiler/cmm/MkZipCfg.hs +5