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

[Anatoly Trosinenko via llvm-commits]

https://github.com/atrosinenko commented:

A few thoughts/suggestions on the tests (just an opinion):
* the test can probably be further simplified by dropping unrelated machine instructions or replacing them with obvious NOPs for readability
* AFAIK in their inline LLVM IR modules, some mir-based tests use dummy function definitions as placeholders - not corresponding to the actual machine instructions (probably, some properties of the original functions have to be kept)
* it may be worth adding simple test cases for the other BTI-clearing instructions

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