[lldb] [ABI/AArch64] Do not add subregs if some of them are present

Fix a bug introduced while refactoring ABIAArch64::AugmentRegisterInfo()
that caused subregisters to be added even if they were already present.
Instead, abort immediately if at least one subregister is found
(following ABIX86).  While at it, add a test for that.

Differential Revision: https://reviews.llvm.org/D111881