[SystemZ] Fix address operand parsing incompatibilities with GAS

The LLVM AsmParser showed different behavior compared to GAS when
parsing address operands in the following two ways:

- If the address operand only has a single register (no comma),
  it is always interpreted as base register by GAS, even in the
  vector-index case (vgef etc.)  This means the following is
  actually incorrect usage, as the base cannot be a vector
  register: vgef %v0, 0(%v1), 0.

- GAS allows specifying a missing base register by using a
  comma after the first register, e.g. vgef %v0, 0(%v1,), 0.

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