[llvm] [ARM][Thumb2] Mark BTI-clearing instructions as scheduling region boundaries (PR #79173)

[Anatoly Trosinenko via llvm-commits]

https://github.com/atrosinenko approved this pull request.

Looks good to me, sorry for long delay. I unresolved a comment that was likely to be resolved unintentionally, though that was a minor issue anyway.

https://github.com/llvm/llvm-project/pull/79173