Remove the free-standing contains_zero_p in favor of the method.

Now that vrange provides a contains_zero_p method, the standalone irange
overload is redundant.  Convert its callers to the method and remove it.
The two-argument REAL_VALUE_TYPE helper is unrelated and stays.

Tested on ppc64le Linux: regstrap and LAPACK.

gcc/ChangeLog:

	* value-range.h (contains_zero_p): Remove the free-standing irange
	overload.
	* range-op.cc (operator_equal::op1_op2_relation): Use the
	irange::contains_zero_p method.
	(operator_not_equal::op1_op2_relation): Likewise.
	(operator_lt::op1_op2_relation): Likewise.
	(operator_le::op1_op2_relation): Likewise.
	(operator_gt::op1_op2_relation): Likewise.
	(operator_ge::op1_op2_relation): Likewise.
	(operator_lshift::op1_range): Likewise.
	(operator_rshift::op1_range): Likewise.
	(operator_cast::op1_range): Likewise.
	(operator_logical_and::fold_range): Likewise.
	(operator_bitwise_and::wi_fold): Likewise.
	(operator_bitwise_xor::op1_range): Likewise.
	(operator_addr_expr::fold_range): Likewise.
	(operator_addr_expr::op1_range): Likewise.
	* range-op-float.cc (operator_equal::op1_op2_relation): Likewise.
	(operator_not_equal::op1_op2_relation): Likewise.
	(operator_lt::op1_op2_relation): Likewise.
	(operator_le::op1_op2_relation): Likewise.
	(operator_gt::op1_op2_relation): Likewise.
	(operator_ge::op1_op2_relation): Likewise.