isl_map.c: extract out shared isl_map_transform
isl_map.c: extract out shared isl_map_transform

This removes some code duplication.
isl_map_transform will also be reused to implement isl_map_range_reverse.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl