RFR: 8318629: G1: Refine code a bit in G1RemSetTrackingPolicy::update_at_allocate

[Hamlin Li]

Hi,
Can you help to review this simple refactoring?
Thanks!

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/16300/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16300&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318629
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16300.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16300/head:pull/16300

PR: https://git.openjdk.org/jdk/pull/16300