RFR: 8344593: GenShen: Review of ReduceInitialCardMarks [v3]

[Y. Srinivas Ramakrishna]

> Fix documentation comment, and add an assertion check upon slowpath allocation.
> 
> I also checked the impact of +/-ReduceInitialCardMarks on GenShen using SPECjbb and didn't see any difference. We've left it enabled by default because less card marking is better in this case.

Y. Srinivas Ramakrishna 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 five additional commits since the last revision:

 - Merge branch 'master' into ricm
 - virtual -> override missed in previous delta.
   Fix zero build (ReduceInitialCardMarks is defined only in
   JVMCI/Compiler2)
 - virtual -> override in derived class ShenandoahBarrierSet.
 - Refine previous change and future-proof ReduceInitialCardMarks for
   GenShen.
 - Fix up documentation comment.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22507/files
  - new: https://git.openjdk.org/jdk/pull/22507/files/23b8103d..76ab8f3b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22507&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22507&range=01-02

  Stats: 10622 lines in 392 files changed: 5385 ins; 3453 del; 1784 mod
  Patch: https://git.openjdk.org/jdk/pull/22507.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22507/head:pull/22507

PR: https://git.openjdk.org/jdk/pull/22507