If I'm interpreting the standard correctly, everything
here was essentially incorrect.
Saying no if fp exceptions are not enabled, then no value is an snan is incorrect.
A loaded value could still be an snan. If FP exceptions
are enabled, certain operations will produce an snan.
Common operations won't produce an snan, and will quiet
an snan input.
Try to get the set of operations that would raise an exception if
they were to produce an snan more correct.