[SystemZ] Bugfix for the 'N' code for inline asm operand.

Don't use a local MachineOperand copy in SystemZAsmPrinter::PrintAsmOperand()
and change the register as it may break the MRI tracking of register
uses. Use an MCOperand instead.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D105757