RFR: 8257793: Shenandoah: SATB barrier should only filter out already strongly marked oops

[Zhengyu Gu]

SATB barrier intercepts oops for later marking, and those oops will be marked as strongly reachable. So that, it can only filter out oops that already strongly marked, not if they are only weakly marked.

- [x] hotspot_gc_shenandoah
- [x] nightly pipeline

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Commit messages:
 - Merge branch 'master' into JDK-8257793-satb-filter
 - JDK-8257793

Changes: https://git.openjdk.java.net/jdk/pull/1655/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1655&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257793
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1655.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1655/head:pull/1655

PR: https://git.openjdk.java.net/jdk/pull/1655