Computing bounds on poly_ints#

poly_int also provides routines for calculating lower and upper bounds:

constant_lower_bound (a): Assert that a is nonnegative and return the smallest value it can have.
constant_lower_bound_with_limit (a, b): Return the least value a can have, given that the context in which a appears guarantees that the answer is no less than b. In other words, the caller is asserting that a is greater than or equal to b even if known_ge (a, b) doesn’t hold.
constant_upper_bound_with_limit (a, b): Return the greatest value a can have, given that the context in which a appears guarantees that the answer is no greater than b. In other words, the caller is asserting that a is less than or equal to b even if known_le (a, b) doesn’t hold.
lower_bound (a, b): Return a value that is always less than or equal to both a and b. It will be the greatest such value for some indeterminate values but necessarily for all.
upper_bound (a, b): Return a value that is always greater than or equal to both a and b. It will be the least such value for some indeterminate values but necessarily for all.