[SystemZ] Improvement of emitMemMemWrapper()

It was discovered that an extra register COPY remained when expanding a
(variable length) memory operation with a loop and there was another use of
the involved address register(s) afterwards.

A simple fix for this is to COPY the address registers before the loop and
use that new vreg instead.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D112065