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

[via llvm-commits]

XChy 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?
If so, I think that's the problem of Sink Pass. I'll try to fix it.

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