RFR: 8327365: Serial: Move _gc_stats to TenuredGeneration [v2]

[Thomas Schatzl]

On Tue, 5 Mar 2024 15:24:02 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Simple refactoring to move a field from superclass to subclass.
>
> Albert Mingkun Yang 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:
> 
>  - review
>  - Merge branch 'master' into s1-flatten-gc-stat
>  - s1-flatten-gc-stat

There is no more `GCStats` use in serial gc, all references to it in `generation.hpp` and `tenuredGeneration.hpp` can be removed (forward decl, include).
Also `GCStats` files can be moved to the `parallel` directory.

Tbh, similar to serial gc, `GCStats` could be replaced by the `AdaptivePaddedNoZeroDevAverage` class in Parallel GC too and completely removed. I.e. imo either both or none of these replacements should be done.

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PR Comment: https://git.openjdk.org/jdk/pull/18122#issuecomment-1980291819