Convert reference mode to unsigned mode before creating an Eor.
Convert reference mode to unsigned mode before creating an Eor.

This fixes opt/fehler296.c.

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