RFR: 8255691: Shenandoah: Invoke native-LRB only on non-strong refs

[Roman Kennke]

The way that current native LRB is implemented is wrong (but non-fatal) and misleading. It's purpose is to prevent resurrection of unreachable non-strong references, and it should only be invoked on non-strong references, not all native references. This distinction will become even more important once we get concurrent reference processing: then we also want to invoke this barrier on referent-loads.

This changes the runtime-part of native-LRB so that it is only invoked when it's invoked with non-strong reference decorator. Otherwise it acts as regular LRB.

Testing: hotspot_gc_shenandoah

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Commit messages:
 - 8255691: Shenandoah: Invoke native-LRB only on non-strong refs

Changes: https://git.openjdk.java.net/jdk/pull/961/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=961&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255691
  Stats: 15 lines in 3 files changed: 4 ins; 3 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/961.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/961/head:pull/961

PR: https://git.openjdk.java.net/jdk/pull/961