aarch64: Remove constraints on sysp operands

The CRn and CRm operands of sysp were unnecessarily constrained to the
ranges C8-C9 and C0-C7.  This constraint has been removed from the
architecture spec, and was never implemented in LLVM, so remove it here
as well.

Additionally, add some more tests to cover the full range of valid sysp
operands, including omitting the pair of optional operands.