edges into dead-end regions.
- Only treat edges *into* dead-end regions as special; edges internal
the region must use the normal rules.
- Conservatively merge information along those edges rather than just
picking one at random. This requires us to not walk into the region
until we've processed all of the edges to it.
- Make sure we prevent *any* stack allocations from being deallocated
if the stack is inconsistent entering a block.
Additionally, fix a bug which was incorrectly treating all blocks that
don't themselves exit the function as ultimately leading to unreachable,
which had inadvertently largely turned off the consistency check.
