Disable tree to bool conversion in frange::update_nan.

Disable tree to bool conversion in frange::update_nan.

We have a set_nan(type) method which can be confused with
update_nan(bool) because of the silent conversion of pointers to bool.
Currently, if you call update_nan(tree), you'll set the possibility of
NAN with a sign of true if tree is non-null.  This is prone to error
and this patch disallows this behavior.

gcc/ChangeLog:

	* value-range.cc (frange::set_nonnegative): Pass bool to
	update_nan.
	* value-range.h: Disallow conversion to bool in update_nan().