[SystemZ] Increase the number of VLREPs

If a loaded value is replicated it is best to combine these two operations
into a VLREP (load and replicate), but isel will not produce this if the load
has other users as well.

This patch handles this by putting the other users of the load to use the
REPLICATE 0-element instead of the load. This way the load has only the
REPLICATE node as user, and we get a VLREP.

Review: Ulrich Weigand
https://reviews.llvm.org/D54264

llvm-svn: 346746