isl_map_simplify.c: drop_unrelated_constraints: avoid affecting copies
isl_map_simplify.c: drop_unrelated_constraints: avoid affecting copies

The input to drop_unrelated_constraints may in theory be a shared
object.  Since the constraints that are being dropped are not redundant,
it is important to not affect any copies of the modified object.

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

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