[llvm] [AArch64] Fix `GCS{PUSHX|POPCX|POPX}` instrs to take an optional register (PR #142121)

[Jonathan Thackray via llvm-commits]

jthackray wrote:

It appears that the published Arm ARM that I have, DDI 0487K.a is actually wrong, and those instructions don't have an optional register operand. This has been fixed in Arm ARM DDI 0487L.

Therefore rejecting this PR.

https://github.com/llvm/llvm-project/pull/142121