RFR: 8344593: GenShen: Review of ReduceInitialCardMarks [v2]

[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 incrementally with three additional commits since the last revision:

 - 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.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22507/files
  - new: https://git.openjdk.org/jdk/pull/22507/files/3bcd441f..23b8103d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22507&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22507&range=00-01

  Stats: 19 lines in 3 files changed: 13 ins; 0 del; 6 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