simplify AND(x >= 0, x < C) --> (unsigned)x < C

Such compares with a signed value are relatively common and can be
easily be simplified into a single unsigned compare. So, do it.

Note: This simplification triggers only 27 times in a x86-64 defconfig
      kernel. I expected more but I suppose it's because most checks
      aren't done against a constant or are done with unsigned values.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>