Fix: don't abandon a backward walk when an unexpandable call is irrelevant
Add first draft of a replacement for the highly complicated load forwarding code
Fix: do check liveness of invariant blocks
Reimplement load forwarding (both normal and PB kinds) using a passive walk instead of inserting fake instructions as before. Preliminary tests indicate this reduces time to perform LF by around 50x.

This commit won't compile -- still need to update prototypes.
Remove the integrator workqueues and associated types made redundant by the new topologically ordered analysis