[9] RFR (XXS): 8062258: compiler/debug/TraceIterativeGVN.java segfaults in trace_PhaseIterGVN

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8062258/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8062258

Trivial fix to avoid NULL dereference w/ -XX:+TraceIterativeGVN while 
querying adr_type() on dead MemNode.

Testing: failing test.

Best regards,
Vladimir Ivanov