[SystemZ] Fix bootstrap failure due to invalid DAG loop

The change in r322988 caused a failure in the bootstrap build bot.
The problem was that directly gluing a BR_CCMASK node to a
compare-and-swap could lead to issues if other nodes were
chained in between.  There is then no way to create a topological
sort that respects both the chain sequence and the glue property.

Fixed for now by rejecting the optimization in this case.  As a
future enhancement, we may be able to handle additional cases
by swapping chain links around.

llvm-svn: 323129