MLBEDSW-5437: MLCE: Vela ignores back2back Activations in model

 - Back-to-back 16-bit activation ops were packed into the same pass
because there was no check to disallow it
 - The solution is to set the appropriate incompatible-flags

Signed-off-by: Tim Hall <tim.hall@arm.com>
Change-Id: Idb3c741a7b52e0d81c1f687f6ecf78352b7872dd