[llvm] [JumpThreading] Thread over BB with only an unconditional branch (PR #86312)

[Quentin Dian via llvm-commits]

https://github.com/DianQK commented:

LGTM with "Comment" :3. (It's usually just because I haven't prepared myself to approve.)

My main concern is the impact of introducing more phi nodes in larger programs. Perhaps I'm overthinking it.
If I remember correctly, #85510 is also affected because `CanRedirectPredsOfEmptyBBToSucc` can only see less facts.

https://github.com/llvm/llvm-project/pull/86312