swift234

[sil-simplify-cfg] Fix a bug in checked_cast_br jump-threading.

If current checked_cast_br is reachable via success, failure and unknown at the same time, then we simply don't know the outcome of the dominating check.
No jump-threading should be performed in this case.

I'll commit the test-case tomorrow. The test-case from the radar depends too much on the stdlib internals. We need something smaller and simpler.

rdar://20389307

Swift SVN r26841