[DAG] canCreateUndefOrPoison - add freeze(assertsext/zext(x,bt)) ->...

[DAG] canCreateUndefOrPoison - add freeze(assertsext/zext(x,bt)) -> assertsext/zext(freeze(x),vt) support

These are guaranteed not to create undef/poison (although they may pass through) - the associated ISD::VALUETYPE node is also guaranteed never to generate poison