RFR: 8265436: G1: Improve gc+phases log output during full gc [v3]

[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

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  Fix error message in test

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3569/files
  - new: https://git.openjdk.java.net/jdk/pull/3569/files/19f5650d..f4137317

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3569&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3569&range=01-02

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