Avoid spurious dep-into-loop barrier if covered by barrier if covered by
barrier inside loop.

In particular, the scheduler will now try to schedule each level of the
tree without boosting before caving and allowing it. If there is a
viable schedule without boosts, boosts are never even tried.

insn.all_inames() -> kernel.insn_inames(insn)
Scheduling:
- Only schedule referenced inames.
- Only work towards insns that have their dependencies satisfied.