RFR: 8300769: Remove G1CollectionSet::_inc_bytes_used_before [v3]

[Thomas Schatzl]

On Fri, 20 Jan 2023 22:26:19 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Thomas Schatzl has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
>> 
>>  - Merge branch 'master' into pull/12113
>>  - ayang review, remove redundant _bytes_used_before
>>  - initial version
>
> I think `_bytes_used` is enough; the `_before` suffix adds nothing and it's confusing -- there's no counterpart like `_bytes_used_after`.

@albertnetymk found that actually the information of `_byte_used_before` can be recreated from `FreeCSetStats`, so `G1CollectionSet::_bytes_used_before` can be removed as well.
Further I fixed the merge errors with latest version.

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PR: https://git.openjdk.org/jdk/pull/12113