[llvm] [AArch64][llvm] Redefine some insns as an alias of `SYS` (PR #187004)

[Jonathan Thackray via llvm-commits]

jthackray wrote:

I'm closing this PR, and splitting it into 2 stacked PRs:
- https://github.com/llvm/llvm-project/pull/203563
- https://github.com/llvm/llvm-project/pull/203564
for easier reviewing

https://github.com/llvm/llvm-project/pull/187004