RFR: JDK-8265259: G1: Fix HeapRegion::block_is_obj for unloading class in full gc [v2]

[Hamlin Li]

> In JDK-8262068, we have introduced an enhancement to skip compacting some heap regions.
> But the objs in these regions might have been dead, and their classes might have been unloaded, at this situation, we need following change to make sure we don't get into trouble when calls HeapRegion::block_is_obj(const HeapWord* p) and subsequent calls e.g. to get obj size.

Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:

  fix one more code path, assert; refine comments.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3514/files
  - new: https://git.openjdk.java.net/jdk/pull/3514/files/7c8562ca..54c04977

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3514&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3514&range=00-01

  Stats: 9 lines in 1 file changed: 2 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3514.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3514/head:pull/3514

PR: https://git.openjdk.java.net/jdk/pull/3514