[llvm] [ARM][Thumb2] Mark BTI-clearing instructions as scheduling region boundaries (PR #87982)

[Victor Campos via llvm-commits]

vhscampos wrote:

This is a follow-up of #79173 after it was reverted.

The only difference is in the testcase, where now it has a `REQUIRES: assert`

https://github.com/llvm/llvm-project/pull/87982