[SystemZ] Don't build a PPA instruction with an immediate 0 operand.
The improvement in the machine verifier for operand types (D63973) discovered
a bad operand in a test using a PPA instruction. It was an immediate 0 where
a register was expected.
This patch fixes this (NFC) by now making the PPA second register operand
NoRegister instead of a zero immediate in the MIR.
Review: Ulrich Weigand