RFR(S): 8209691: Allow MemBar on single memory slice

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8209691/webrev.00/

This change propagates MemBarNode::_adr_type after matching. It's then
possible to have a membar on only a single slice. This is another change
from the shenandoah repo where we need membars in a common code pattern
and don't want a wide membar to disrupt most memory related
optimizations.

Roland.