Do not clobber signbit when unioning a NAN.

Do not clobber signbit when unioning a NAN.

When unioning a known NAN and something else, we're dropping the
properties of the NAN, particularly the sign.  This fixes the
oversight.

With this patch, we should be keeping the sign bit up to date, even in
the presence of NANs.

gcc/ChangeLog:

	* value-range.cc (frange::union_): Do not drop properties when
	unioning a NAN with something else.
	(range_tests_signed_zeros): Add tests.