[llvm] [MachineBasicBlock] Don't split loop header successor if the terminator is unanalyzable (PR #170146)

[Artem Belevich via llvm-commits]

https://github.com/Artem-B approved this pull request.

LGTM in principle, but needs a few more code comments.

https://github.com/llvm/llvm-project/pull/170146