frange: Fix up foperator_{,not_}equal::fold_range for signed zeros [PR108540]

The following testcases are miscompiled, because threader sees some
SSA_NAME would have -0.0 value and when computing range of SSA_NAME == 0.0
foperator_equal::fold_range sees one operand has [-0.0, -0.0] singleton
range, the other [0.0, 0.0], they aren't equal (frange operator== uses
real_identical etc. rather than real comparisons) and so it thinks they
compare unequal.  With signed zeros -0.0 == 0.0 is true though, so we
need to special case the both ranges singleton code.
Similarly, if we see op1 range being say [-42.0, -0.0] and op2 range
[0.0, 42.0], we'd check that the intersection of the two ranges is empty
(that is correct) and fold the result of == between such operands to
[0, 0] which is wrong, because -0.0 == 0.0, it needs to be [0, 1].
Similarly for foperator_not_equal::fold_range.

2023-01-26  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/108540
	* range-op-float.cc (foperator_equal::fold_range): If both op1 and op2
	are singletons, use range_true even if op1 != op2
	when one range is [-0.0, -0.0] and another [0.0, 0.0].  Similarly,
	even if intersection of the ranges is empty and one has
	zero low bound and another zero high bound, use range_true_and_false
	rather than range_false.
	(foperator_not_equal::fold_range): If both op1 and op2
	are singletons, use range_false even if op1 != op2
	when one range is [-0.0, -0.0] and another [0.0, 0.0].  Similarly,
	even if intersection of the ranges is empty and one has
	zero low bound and another zero high bound, use range_true_and_false
	rather than range_true.

	* gcc.c-torture/execute/ieee/pr108540-1.c: New test.
	* gcc.c-torture/execute/ieee/pr108540-2.c: New test.

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