isl_vertices.c: add_vertex: return isl_bool indicating whether vertex was added
isl_vertices.c: add_vertex: return isl_bool indicating whether vertex was added

The function add_vertex currently always adds the vertex and therefore
always returns isl_bool_true (unless an error has occurred).
However, changing the meaning of the return type will allow
add_vertex to reject some vertices in the next commit.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

Meinersbur/isl,UBERTC/isl,PollyLabs/isl,PollyLabs/isl,UBERTC/isl,inducer/isl-mirror,Meinersbur/isl,inducer/isl-mirror,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,tobig/isl,inducer/isl-mirror,tobig/isl,nicolasvasilache/isl,tobig/isl,PollyLabs/isl,inducer/isl-mirror,nicolasvasilache/isl,UBERTC/isl,UBERTC/isl,nicolasvasilache/isl,nicolasvasilache/isl,inducer/isl-mirror,nicolasvasilache/isl,tobig/isl,Meinersbur/isl