[llvm] [AArch64][PAC] Refine authenticated pointer check methods (PR #74074)

[Kristof Beyls via llvm-commits]

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

These changes make sense to me.
I'm assuming that aligning the values of the brk operand with what arm64e does helps reduce possible incompatibilities with any users/tools that might interpret these brk operand values?
Or is there another rationale to align the brk operand values with what arm64e does?
I didn't try to check independently if these are indeed the values arm64e uses.

https://github.com/llvm/llvm-project/pull/74074