vrp: Fix operator_trunc_mod::op1_range [PR97888]

vrp: Fix operator_trunc_mod::op1_range [PR97888]

As mentioned in the PR, in (x % y) >= 0 && y >= 0, we can't deduce
x's range to be x >= 0, as e.g. -7 % 7 is 0.  But we can deduce it
from (x % y) > 0.  The patch also fixes up the comments.

2020-11-18  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/91029
	PR tree-optimization/97888
	* range-op.cc (operator_trunc_mod::op1_range): Only set op1
	range to >= 0 if lhs is > 0, rather than >= 0.  Fix up comments.

	* gcc.dg/pr91029.c: Add comment with PR number.
	(f2): Use > 0 rather than >= 0.
	* gcc.c-torture/execute/pr97888-1.c: New test.
	* gcc.c-torture/execute/pr97888-2.c: New test.