RFR: 8280885: Shenandoah: Some tests failed with "EA: missing allocation reference path"

[Roland Westrelin]

EA encounters an unexpected IU barrier. I think it's good enough to
change ShenandoahBarrierSetC2::is_gc_barrier_node() so it returns true
for IU barriers.

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Commit messages:
 - fix & test

Changes: https://git.openjdk.java.net/jdk/pull/7299/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7299&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280885
  Stats: 75 lines in 2 files changed: 74 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7299.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7299/head:pull/7299

PR: https://git.openjdk.java.net/jdk/pull/7299