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

[Jonathan Thackray via llvm-commits]

jthackray wrote:

> So you kept all these 4 as instructions which is wrong GCSPUSHM, GCSSS1, GCSPOPM, GCSSS2. Sys is already marked as mayLoad/Store and hasSIdeEffects and all other instructions should be marked like that as well. This is because we can't really tell what they will do. GCSSS2 can be resolved by lowering into sys instruction and GCSPOPM can be resolved by creating the pattern for sys instead.

Thanks @Lukacma. I've tried to do this for these 4 instructions; seems to be okay.

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