[AArch64][llvm] Redefine some isns as an alias of `SYS`
Some instructions are not currently defined as an alias of `SYS`
when they should be, so they don't disassemble back into the
native instruction, but instead disassemble into `SYS`.
Fix these cases and add additional testcase.
Note that I've left `GCSPUSHM` due to a `mayStore`, `GCSSS1` and
`GCSSS2` as they're used in AArch64ISelDAGToDAG.cpp, and `GCSPOPM`
has an intrinsic pattern in AArch64InstrInfo.td. They will disassemble
correctly though, as they use `InstAlias`.
