Improved optimization of division by constants.
Improved optimization of division by constants.

This implements the "round down" algorithm by ridiculous_fish (see
Faster Unsigned Division by Constants,
http://ridiculousfish.com/blog/posts/labor-of-division-episode-iii.html),
which improves the generated code for some constants (like 42).

libfirm/libfirm,killbug2004/libfirm,8l/libfirm,libfirm/libfirm,libfirm/libfirm,killbug2004/libfirm,8l/libfirm,davidgiven/libfirm,jonashaag/libfirm,MatzeB/libfirm,davidgiven/libfirm,jonashaag/libfirm,8l/libfirm,davidgiven/libfirm,killbug2004/libfirm,davidgiven/libfirm,davidgiven/libfirm,jonashaag/libfirm,8l/libfirm,davidgiven/libfirm,jonashaag/libfirm,8l/libfirm,MatzeB/libfirm,killbug2004/libfirm,killbug2004/libfirm,libfirm/libfirm,killbug2004/libfirm,jonashaag/libfirm,davidgiven/libfirm,MatzeB/libfirm,8l/libfirm,jonashaag/libfirm,MatzeB/libfirm,libfirm/libfirm,jonashaag/libfirm,8l/libfirm,MatzeB/libfirm,killbug2004/libfirm,MatzeB/libfirm,MatzeB/libfirm