isl_mat.c: extract out shared check_row
isl_mat.c: extract out shared check_row

This function will also be reused in the next commit.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

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