Fix r15800: a & (a ^ b) is a & ~b, not a & -b.
Fix r15800: a & (a ^ b) is a & ~b, not a & -b.

[r15874]

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