Before this commit, I tried to make sure that
two interfering temporaries never ended up in
the same phi class.
This was to make sure that their register hints
were not counterproductively stepping on each
other's toes. The idea is fine, but:
* the implementation is clumsy because it
mixes the orthogonal concepts of
(i) interference and (ii) phi classes;
* the hinting process in the register
allocator is hard to understand because
the disjoint-set data structure used for
phi classes is cut in arbitrary places.
After this commit, the phi classes *really* are
phi classes represented with a proper disjoint-set
data structure.
