/* merge spillslots and interferences */
res = uf_union(spillslot_unionfind, s1, s2);
/* we assume that we always merge s2 to s1 so swap s1, s2 if necessary */
/* merge spillslots and interferences */
res = uf_union(spillslot_unionfind, s1, s2);
/* we assume that we always merge s2 to s1 so swap s1, s2 if necessary */