This patch makes value_range_base::set convert POLY_INT_CST bounds
into the worst-case INTEGER_CST bounds.  The main case in which this
gives useful ranges is a lower bound of A + B * X becoming A when B >= 0.
E.g.:

  [32 + 16X, 100] -> [32, 100]
  [32 + 16X, 32 + 16X] -> [32, MAX]

But the same thing can be useful for the upper bound with negative
X coefficients.

2019-10-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR middle-end/92033
	* poly-int.h (constant_lower_bound_with_limit): New function.
	(constant_upper_bound_with_limit): Likewise.
	* doc/poly-int.texi: Document them.
	* tree-vrp.c (value_range_base::set): Convert POLY_INT_CST bounds
	into the worst-case INTEGER_CST bounds.

From-SVN: r277056