RFR(S): 8211233: MemBarNode::trailing_membar() and MemBarNode::leading_membar() need to handle dying subgraphs better

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8211233/webrev.00/

I hit a bug where MemBarNode::leading_membar() doesn't return the right
result because a dying part of the graph between a trailing and a
leading membars is not properly handled.

Roland.