RFR: 8254320: Shenandoah: C2 native LRB should activate for non-cset objects [v2]

[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.

Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev
excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since
the last revision:

 - Merge remote-tracking branch 'upstream/master' into 8254320
 - 8254320: Shenandoah: C2 native LRB should activate for non-cset objects

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/580/files
  - new: https://git.openjdk.java.net/jdk/pull/580/files/bd11f8b1..05b2bee3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=580&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=580&range=00-01

  Stats: 1323 lines in 15 files changed: 1278 ins; 4 del; 41 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