isl_input.c: construct_range: break up into smaller functions
isl_input.c: construct_range: break up into smaller functions

These functions will be reused in the next commit.

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

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