RFR: 8265436: G1: Improve gc+phases log output during full gc

[Thomas Schatzl]

Hi all,

  can I have reviews for this fix to remove recently introduced log messages from `debug` to `trace` level?

If you select `gc+phases=debug` logging with [JDK-8262068](https://bugs.openjdk.java.net/browse/JDK-8262068), you get:

    [0.695s][debug][gc,phases] GC(51) Phase 1: Weak Processing 0.309ms
    [0.695s][debug][gc,phases] GC(51) ClassLoaderData 0.003ms
    [0.695s][debug][gc,phases] GC(51) Trigger cleanups 0.002ms
    [0.699s][debug][gc,phases] GC(51) Phase 1: Class Unloading and Cleanup 4.056ms
    [0.699s][info ][gc,phases] GC(51) Phase 1: Mark live objects 7.722ms
    [0.699s][debug][gc,phases] GC(51) Phase 2: skip compaction region index: 0, live words: 125610
    [repeated for every region skipped] 

the last per-region message is quite annyoing and just fills up logs as the code prints one line per affected region (which may be a lot), so I suggest to move it to `trace` level. I did suggest to add some nice phase 2 region summary in [JDK-8265437](https://bugs.openjdk.java.net/browse/JDK-8265437), but that is out of scope for this change.

Testing: local testing

Thanks,
  Thomas

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Commit messages:
 - Initial change

Changes: https://git.openjdk.java.net/jdk/pull/3569/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3569&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265436
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3569.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3569/head:pull/3569

PR: https://git.openjdk.java.net/jdk/pull/3569