[llvm] [JumpThreading] Remove deleted BB from Unreachable (PR #126984)

[via llvm-commits]

weiguozhi wrote:

> > As you have noticed MergeBasicBlockIntoOnlyPred can mark an unreachable block for deletion. The deletion is deferred until DTU->flush() is called, then the unreachable BB is actually deleted, but the set Unreachable is not modified, so a deleted BB pointer is left in Unreachable.
> 
> I think the last bit I'm still missing is when JumpThreading calls DTU->flush() _during_ the transform. I thought JT only flushes DT updates after everything is done, at which point the Unreachable set is not relevant anymore.

DTU->flush() is called in the chain:
```
    threadThroughTwoBasicBlocks()
->  getOrCreateBFI()
->  runExternalAnalysis<BlockFrequencyAnalysis>()
->  DTU->flush()

```

https://github.com/llvm/llvm-project/pull/126984