improve isl_pw_aff_list_{min,max} implementation
improve isl_pw_aff_list_{min,max} implementation

The original implementation would first combine two elements and
then continue to combine the result with an extra element in each step.
For a list of n elements this results in a subdivisions
into a number of pieces that is in the order of 2^n.
The new implementation directly computes the result
taking into account all elements and only requires a subdivision
into n pieces.

Reviewed-by: Tianjiao Sun <07f9c69eb0c44438de986fa0c33d1e083f209b1b@cerebras.net>
Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

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