isl_map_coalesce: add a note about a possible improvement wrt extensions
isl_map_coalesce: add a note about a possible improvement wrt extensions

The possible improvement has not been implemented yet because
no test case was found so far where it would be useful.

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

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