RFR: 8254320: Shenandoah: C2 native LRB should activate for non-cset objects [v3]

[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 incrementally with one additional commit since the last revision:

  Add comment why native-LRB doesn't need the cset-check

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/580/files
  - new: https://git.openjdk.java.net/jdk/pull/580/files/05b2bee3..e5b1e89e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=580&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=580&range=01-02

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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