Switch the BB CFG cache from postorder to RPO
The `BasicBlocks` CFG cache is interesting:
- it stores a postorder, but `traversal::postorder` doesn't use it
- `traversal::reverse_postorder` does traverse the postorder cache backwards
- we do more RPO traversals than postorder traversals (around 20x on the perf.rlo benchmarks IIRC) but it's not cached
- a couple places here and there were manually reversing the non-cached postorder traversal
This PR switches the order of the cache, and makes a bit more use of it. This is a tiny win locally, but it's also for consistency and aesthetics.
r? `@ghost`
677710eaf0