arch64: Add optional operand register pair support tests

arch64: Add optional operand register pair support tests

Add tests to cover the full range of behaviors observed around
optional register operands for the `tlbip' and `sysp' instructions,
namely:

  * Not all `tlbip' operations take GPR operands.  When this is the
  case, we should check that neither optional operand was supplied.
  * When a `tlbip' operation is labeled with the `F_HASXT' flag, xzr
  is not a valid optional operand.  In such case, at least the fist
  optional register needs to be specified with a non-xzr value.
  * The first operand for both insns should be either xzr or an
  even-numbered register (n % 2 == 0).  In the former scenario, the
  second operand should default to xzr too, while in the latter, it
  should default to n + 1.