Reconcile handling of EH vars (#1884)

Reconcile handling of EH vars (#1884)

* Reconcile handling of EH vars

Modify `BasicBlock::hasEHBoundaryIn()` and `BasicBlock::hasEHBoundaryOut()` to reflect what liveness is doing, and adjust the register allocator code appropriately.

It might make sense to extract the code that's basically the same, except that the register allocator doesn't need the `finallyVars` except as validation in the `DEBUG` case.

Fix #1786