[llvm] [JumpThreading] Fix bugs of simplifying BB with unreachable predecessor (PR #68576)

[Nikita Popov via llvm-commits]

nikic wrote:

> > Unreachable blocks are dominated by all blocks, so this is well-defined IR. If a pass fails to handle it correctly, that pass needs to be fixed.
> 
> Does it mean `value` in `phi [%value, %unreachable]` can be everything defined?

Yes, that's right.

https://github.com/llvm/llvm-project/pull/68576