RFR: 8349766: GenShen: Bad progress after degen does not always need full gc [v2]

[Kelvin Nilsen]

> In generational mode, only upgrade to full GC from degenerated GC if we've done two degenerated cycles in a row and both indicated bad progress.  Otherwise, start another concurrent GC, which will most likely degenerate also.  But this degenerated cycle will reclaim floating garbage within the young generation much more quickly than a full GC would have done.

Kelvin Nilsen has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - 8348092: Shenandoah: assert(nk >= _lowest_valid_narrow_klass_id && nk <= _highest_valid_narrow_klass_id) failed: narrowKlass ID out of range (3131947710)
   
   Reviewed-by: shade
 - Merge tag 'jdk-25+10' into defer-generational-full-gc
   
   Added tag jdk-25+10 for changeset a637ccf2
 - Be less eager to upgrade degen to full gc

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23552/files
  - new: https://git.openjdk.org/jdk/pull/23552/files/17e5e919..8d662e10

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23552&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23552&range=00-01

  Stats: 9730 lines in 593 files changed: 6779 ins; 1405 del; 1546 mod
  Patch: https://git.openjdk.org/jdk/pull/23552.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23552/head:pull/23552

PR: https://git.openjdk.org/jdk/pull/23552