Added hook fir node substitution during dead node elimination
Added hook fir node substitution during dead node elimination

[r7416]

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