[AArch64][SME][NFC] Add implicit operands for SME instructions in the disassembly.

This patch simplifies the switch statement in getInstruction to add
implicit operands (register ZA and Immediate  equal to zero)
in the SME operands when disassembly.

The register ZA and the zero immediate  can be added by checking the operand
in MCInstDesc.

Differential Revision: https://reviews.llvm.org/D125534