[SystemZ] Implement getRepRegClassFor()

This method must return a valid register class, or the list-ilp isel
scheduler will crash. For MVT::Untyped nullptr was previously returned, but
now ADDR128BitRegClass is returned instead. This is needed just as long as
list-ilp (and probably also list-hybrid) is still there.

Review: Ulrich Weigand, A Trick
https://reviews.llvm.org/D32802

llvm-svn: 302649