the Supremacy clause exists to make it clear that the Constitution is supreme to all treaties or laws ( existing or future laws/treaties)
I don't believe the 8th comes into play when we are talking torture of unlawful combatants.... EIT/torture is not a punishment
if the action ( torture) is intended to cause pain/mental anquish, etc.... it would be considered torture and would be unlawful ( not necessarily unconstitutional)
if it is intended to gather information, it can be lawful .
our own law( US code).. and that of the UN.. stipulates that intent is a component to determine whether or not an action is torture.