fold-const: Fix up make_range_step [PR105189]

fold-const: Fix up make_range_step [PR105189]

The following testcase is miscompiled, because fold_truth_andor
incorrectly folds
(unsigned) foo () >= 0U && 1
into
foo () >= 0
For the unsigned comparison (which is useless in this case,
as >= 0U is always true, but hasn't been folded yet), previous
make_range_step derives exp (unsigned) foo () and +[0U, -]
range for it.  Next we process the NOP_EXPR.  We have special code
for unsigned to signed casts, already earlier punt if low or high
aren't representable in arg0_type or if it is a narrowing conversion.
For the signed to unsigned casts, I think if high is specified we
are still fine, as we punt for non-representable values in arg0_type,
n_high is then still representable and so was smaller or equal to
signed maximum and either low is not present (equivalent to 0U), or
low must be smaller or equal to high and so for unsigned exp
+[low, high] the signed exp +[n_low, n_high] will be correct.
Similarly, if both low and high aren't specified (always true or
always false), it is ok too.
But if we have for unsigned exp +[low, -] or -[low, -], using
+[n_low, -] or -[n_high, -] is incorrect.  Because low is smaller
or equal to signed maximum and high is unspecified (i.e. unsigned
maximum), when signed that range is a union of +[n_low, -] and
+[-, -1] which is equivalent to -[0, n_low-1], unless low
is 0, in that case we can treat it as [-, -].

2022-04-08  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/105189
	* fold-const.c (make_range_step): Fix up handling of
	(unsigned) x +[low, -] ranges for signed x if low fits into
	typeof (x).

	* g++.dg/torture/pr105189.C: New test.

(cherry picked from commit 5e6597064b0c7eb93b8f720afc4aa970eefb0628)