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
Be more careful when replacing targets of terminators. This finally implements a long-due FIXME in CheckedCastBrJumpThreading.
rdar://20345557.
Swift SVN r26725
