isl_map.c: extract out shared isl_map_unmark_normalized
isl_map.c: extract out shared isl_map_unmark_normalized

This will be reused in the next commit.

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

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