Changes to ghc

Changes to ghc:
commit 6b2ccadc2575aecc7ed0c6a1991c21a09dd4a1e4
Author: Iavor S. Diatchki <iavor.diatchki@gmail.com>
Date:   Tue Oct 15 09:43:39 2013 -0700

    Weaken the improvement for subtraction.

    For details see the comment on `interactTopSub`.