RFR: 8254320: Shenandoah: C2 native LRB should activate for non-cset objects

[Roman Kennke]

In C2 native-LRB, we don't call the slow-path when the reference is not in cset. However, it may need to return NULL
for non-cset objects too, in order to prevent accidental ressurrection of referenced object.

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Commit messages:
 - 8254320: Shenandoah: C2 native LRB should activate for non-cset objects

Changes: https://git.openjdk.java.net/jdk/pull/580/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=580&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254320
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/580.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/580/head:pull/580

PR: https://git.openjdk.java.net/jdk/pull/580