RFR: Broaden plab region search

[Kelvin Nilsen]

Recent testing revealed that old-gen heap regions were being ignored in the search to satisfy new PLAB allocation requests.  It was discovered that many of these regions are found within ranges of the ShenandoahFreeSet that are not considered to be "is_collector_free()".

This is a first step in a two-step change.  In this patch, we broaden the search for old-gen heap regions that have available memory to include regions that are not is_collector_free.  In a second step of this improvement, we intend to restructure the implementation of the ShenandoahFreeSet to better distinguish ranges of regions that hold young-gen survivors, which ranges hold old-gen, and which ranges are intended to serve as mutator allocations.

On an Extremem workload that allocates roughly 626 M/s in a total heap size of 49G with an old-gen usage of 18.7G, we saw significant improvements compared to mainline generational Shenandoah implementation:

1. Concurrent GC passes decreased from 606 to 493 (19% improvement)
2. Degenerated GC passes decreased from 17 to 3 (82% improvement)
3. Full GCs decreased from 15 to 3 (80% improvement)
4. P50 latency for Customer Preparation Processing (CPP) improved from 1798 us to 1735 us (3.5%)
5. P100 latency for CPP improved from 25_636_285 us to 9_148_580 us (64% improvement)

Across a broad assortment of performance related CI tests, we also benefits on x86:

-74.24% extremem-phased/do_nothing_p99 p=0.00061
  Control:      2.318s  (+/-941.25ms)         80
  Test:         1.330s  (+/-  1.01s )         15

-15.70% extremem-phased/context_switch_count p=0.02032
  Control:  28188.234   (+/-5868.23  )         80
  Test:     24362.538   (+/-4260.19  )         15

-6.26% extremem-phased/do_nothing_p50 p=0.00246
  Control:    603.203us (+/- 38.32us)         80
  Test:       567.692us (+/- 50.34us)         15

And on aarch64:

+22.92% specjbb2015/sla_10000_jops p=0.01104
  Control:   2607.153   (+/-799.74  )         90
  Test:      3204.615   (+/-592.15  )         15

-5.85% extremem-phased/do_nothing_p50 p=0.00675
  Control:    608.153us (+/- 44.52us)         90
  Test:       574.538us (+/- 47.49us)         15

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

Commit messages:
 - Fix white space
 - Remove instrumentation
 - Fix my fix limiting find-next-marked-object
 - Fix request to find next marked
 - Enhance log messages for generations at end of gc
 - Allow the search for old-gen PLAB to see regions not collector-free

Changes: https://git.openjdk.org/shenandoah/pull/198/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=198&range=00
  Stats: 66 lines in 6 files changed: 54 ins; 4 del; 8 mod
  Patch: https://git.openjdk.org/shenandoah/pull/198.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/198/head:pull/198

PR: https://git.openjdk.org/shenandoah/pull/198