RFR: 8372861: Genshen: Override parallel_region_stride of ShenandoahResetBitmapClosure to a reasonable value for better parallelism

[Xiaolong Peng]

In concurrent reset/concurrent reset after collect phase, the worker needs to reset bitmaps for all the regions in current GC generation. The problem is resetting bitmaps may takes long for large heap because the marking bitmaps are also larger than small heap, we should always consider multiple threads if there are more than concurrent workers for concurrent reset. 

In this PR, parallel_region_stride for ShenandoahResetBitmapClosure is set to 8 for best possible workload distribution to all active workers.

Test result:

java -XX:+TieredCompilation -XX:+AlwaysPreTouch -Xms32G -Xmx32G -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:+UnlockDiagnosticVMOptions -Xlog:gc* -XX:-ShenandoahUncommit -XX:ShenandoahGCMode=generational  -XX:+UseTLAB -jar ~/Downloads/dacapo-23.11-MR2-chopin.jar -n 5 h2 | grep "Concurrent Reset"

With the change:

[77.867s][info][gc,stats    ] Concurrent Reset               =    0.043 s (a =     3039 us) (n =    14) (lvls, us =     1133,     1230,     1270,     1328,    14650)
[77.867s][info][gc,stats    ] Concurrent Reset After Collect =    0.043 s (a =     3107 us) (n =    14) (lvls, us =     1094,     1230,     1855,     3457,     8348)

Original:


[77.289s][info][gc,stats    ] Concurrent Reset               =    0.045 s (a =     3197 us) (n =    14) (lvls, us =     1172,     1191,     1309,     1426,    15582)
[77.289s][info][gc,stats    ] Concurrent Reset After Collect =    0.105 s (a =     7476 us) (n =    14) (lvls, us =     2246,     3828,     4395,     7695,    21266)


The average time of concurrent reset after collect is reduced from 7476 us to 3107 us, 100%+ improvement.

### Other tests
- [x] hotspot_gc_shenandoah

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

Commit messages:
 - Fix wrong impl of parallel_region_stride in ShenandoahExcludeRegionClosure & ShenandoahIncludeRegionClosure
 - Add comments
 - Set parallel_region_stride to 8 for ShenandoahResetBitmapClosure
 - Tidying
 - Override ShenandoahParallelRegionStride to 8 when wrap the closure with ShenandoahIncludeRegionClosure
 - Override ShenandoahParallelRegionStride to 8 when wrap the closure with ShenandoahExcludeRegionClosure

Changes: https://git.openjdk.org/jdk/pull/28613/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28613&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372861
  Stats: 16 lines in 4 files changed: 15 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28613.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28613/head:pull/28613

PR: https://git.openjdk.org/jdk/pull/28613