ia32: Use an enum bitfield for the attribute tp of ia32_attr_t.
ia32: Use an enum bitfield for the attribute tp of ia32_attr_t.

* Also only use the necessary number of bits.
* Improve its comment.

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