Given the current `OPT()` node:
- `optimize(compressor)` works on the same level, and will only `descend()` if returned instance is different
- `transform(compressor)` works on child nodes, and will always `descend()`
Other fixes
- `loopcontrol_target()` should compare against `compressor.self()` since the current node may have already been displaced
- remove obsolete optimisation in `AST_Binary` after #1477fixes#1592
