[SystemZ] Improve verification of MachineOperands.

Now that the machine verifier will check for cases of register/immediate
MachineOperands and their correspondence to the MC instruction descriptor,
this patch adds the operand types to the descriptors where they were
previously missing. All MCOI::OPERAND_UNKNOWN operand types have been handled
to get a known type, except for G_... (global isel) instructions.

Review: Ulrich Weigand
https://reviews.llvm.org/D71494