isl_test.c: test_dim_max_1: extract out check_single_piece
isl_test.c: test_dim_max_1: extract out check_single_piece

This function will be reused in the next commit.

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

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