RFR: 8300769: Remove G1CollectionSet::_inc_bytes_used_before [v4]

[Thomas Schatzl]

> Hi all,
> 
>   please review this removal of `G1CollectionSet::_inc_bytes_used_before`; this variable holds the amount of bytes used in eden regions, updated during mutator time. At the start of gc its value is copied to `G1CollectionSet::_bytes_used_before`, and then never touched again. The `_bytes_used_before` is read at end of gc, and then reset.
> 
> This change simply removes `_inc_bytes_used_before` and directly uses `_bytes_used_before` to accumulate eden region used bytes.
> 
> Testing: gha
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  ayang review2

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12113/files
  - new: https://git.openjdk.org/jdk/pull/12113/files/cbf4de63..7220ffa5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12113&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12113&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12113.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12113/head:pull/12113

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