fixed optimize_conv_conv(): when the result is 8bit, we must change the opcode AND register constraints
fixed optimize_conv_conv(): when the result is 8bit, we must change the opcode AND register constraints

[r15569]

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