RFR: 8279910: G1: Simplify HeapRegionRemSet::add_reference

[Hamlin Li]

HeapRegionRemSet::add_reference checks "state == Untracked" and return if true; it's called at G1RebuildRemSetClosure::do_oop_work and G1ConcurrentRefineOopClosure::do_oop_work respectively.

The check in HeapRegionRemSet::add_reference could be replaced with an assert, and move the check up to one of the caller G1RebuildRemSetClosure::do_oop_work.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/7045/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7045&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279910
  Stats: 7 lines in 2 files changed: 2 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7045.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7045/head:pull/7045

PR: https://git.openjdk.java.net/jdk/pull/7045