Validate that selections are structured (#2962)

* Validate that selections are structured

WIP

* new checks that switch and conditional branch are proceeded by a
selection merge where necessary

* Don't consider unreachable blocks

* Add some tests

* Changed how labels are marked as seen

* Moved check to more appropriate place
* Labels are now marked as seen when there are encountered in a
terminator instead of when the block is checked
* more tests

* more tests

* Method comment

* new test for a bad case