G1: Improve gc+phases log output during full gc

After JDK-8262068 gc+phases=debug output during full gc looks as follows:

[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]

Since heaps can have thousands of regions, this is too much information at this log level.

Move these per-region messages to gc+phases=trace or somewhere more appropriate.