Switch tests from using `-debug[-only=LoopVectorize]` to
`-vplan-print-after` as that provides better control at what step in the
pipeline we want to check the VPlan (I'm using `optimize$` for now to
preserve previous state).
Then, update `-vplan-print-after*` to print what function the loop
belongs to. That enables us to simplify VPlan UTC support as the output
of the updated tests contains the VPlan dump only - no special
filtering/extraction is necessary anymore.
8e335d5336