isl_ast_build_expr.c: every_non_zero_coefficient: allow reverse order
isl_ast_build_expr.c: every_non_zero_coefficient: allow reverse order

That is, allow coefficients to be traversed in reverse order.
This is needed for the next commit.

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

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