RFR: 8255999: Shenandoah: Invoke native-LRB only on non-strong refs, again

[Roman Kennke]

In JDK-8255691, I changed the condition for invoking native-LRB only on non-strong-refs. Somehow, when merging JDK-8254315, I got the condition wrong again. 

Testing: hotspot_gc_shenandoah

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Commit messages:
 - 8255999: Shenandoah: Invoke native-LRB only on non-strong refs, again

Changes: https://git.openjdk.java.net/jdk/pull/1101/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1101&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255999
  Stats: 11 lines in 1 file changed: 6 ins; 4 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1101.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1101/head:pull/1101

PR: https://git.openjdk.java.net/jdk/pull/1101