isl_ast_expr_from_constraint: split off constant term if it avoids operation

isl_ast_expr_from_constraint: split off constant term if it avoids operation

If one hand of the comparison operation that is constructed would be
exactly zero, then we put the constant term on that side.
This avoids the operation that adds the constant term to the other side,
resulting in slightly smaller expressions that are easier to read and
that may also reduce the risk of overflows.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>