beflags: move be_Keep logic out of is_modify_flags function where it didn't really belong
beflags: move be_Keep logic out of is_modify_flags function where it didn't really belong

[r27818]

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