Add contains_zero_p to the vrange hierarchy.

Introduce a contains_zero_p() predicate on vrange and company,
returning whether the range contains zero.  It is a clearer
replacement for nonzero_p(), which only holds for the exact ~[0,0]
range.

Tested on ppc64le Linux.

gcc/ChangeLog:

	* value-range.h (vrange::contains_zero_p): New pure virtual.
	(unsupported_range::contains_zero_p): Declare.
	(irange::contains_zero_p): Declare and define.
	(prange::contains_zero_p): Declare and define.
	(frange::contains_zero_p): Declare.
	(Value_Range::contains_zero_p): New.
	* value-range.cc (unsupported_range::contains_zero_p): Define.
	(frange::contains_zero_p): Define.