isl_input.c: read_tuple_pw_aff_el: extract out space_set_last_dim_name
isl_input.c: read_tuple_pw_aff_el: extract out space_set_last_dim_name

This will be reused in the next commit.

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

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