Under ithreads, convert SVOPs stored in MADPROPs to PADOPs.

Under ithreads, convert SVOPs stored in MADPROPs to PADOPs.

Else if a child thread attempts to free an optree with MADPROPs containing OPs
pointing directly to SVs, it will by trying to free SVs to the wrong
interpreter, at which point bad things(tm) happen.

(There still seems to be some fixing needed for the MADPROPs direct pointers,
but as no tests are failing because of them, I'm postponing them until the
failures are addressed)