Reinstate some aspects of the PB analyser:

* Do erase PBs before considering a loop, because of the possibility that there are subcalls that have attempted PB analysis at their own scope. At the moment this is only possible when --int-ignore-loop is in effect.
* Consider PB analysis even for !isUnresolved instructions, since the PB analyser assumes that e.g. GEP of known object will be eval'd, whilst conventional LF explores such relationships every time it encounters them.
* Do promote single-value PBs to "proper" known values.