isl_basic_map_extend_dim: move responsibility of cow'ing to the caller

isl_basic_map_extend_dim: move responsibility of cow'ing to the caller

Some callers, notably isl_basic_map_detect_equalities, want to
extend the basic map to add equalities that are implicit to the
basic map.  These equalities should be added to all copies,
so we definitely do not want to cow in such cases.