isl_tab_basic_map_partial_lexopt: use context constraints when detecting equalities

isl_tab_basic_map_partial_lexopt: use context constraints when detecting equalities

The constraints of the context of the actual map may conspire to form
equalities and we generally want to detect as many equalities as possible.
Therefore, detect equalities in the intersection of the two
and add these equalities to the map.
Note that the equalities are now no longer detected in the map
itself, so we may have to detect equalities again later if
we are ever interested in the equalities of the map itself.