Fix DCE for ownership forwarding instructions
DCE deletes ownership forwarding instructions when it doesn’t have useful users.
It inserts destroy_value/end_borrow for its operands to compensate their lifetimes.
DCE also deletes branches when its successor blocks does not have useful instructions.
It deletes blocks and creates a jump to the nearest post dominating block.
When DCE needs to delete a forwarding instruction in a dead block, it cannot just create
lifetime ends of its operands at its position. Use LifetimeCompletion utility in such cases.
rdar://140428721
