This patch fixes incorrect results for HOST_WIDE_INT positions
at opposite extremes when used with HOST_WIDE_INT sizes.  It also
fixes UB when comparing such positions with unsigned HOST_WIDE_INT
sizes (although the results in that case were wrapv-correct).

2018-03-20  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/84811
	* poly-int.h (poly_span_traits): Remove the T3 parameter and
	promote HOST_WIDE_INT T2 - T1 results to unsigned HOST_WIDE_INT.
	(maybe_in_range_p, known_in_range_p, ranges_known_overlap_p):
	(known_subrange_p): Update accordingly.  Cast each value involved
	in the size comparison, rather than casting the result of the
	subtraction.

gcc/testsuite/
	PR tree-optimization/84811
	* gcc.dg/torture/pr84811.c: New test.

From-SVN: r258743