Improve scheduling: ignore keeps, sort roots by height.
Improve scheduling: ignore keeps, sort roots by height.

[r19563]

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