RFR: 8300769: Remove G1CollectionSet::_inc_bytes_used_before

[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

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Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/12113/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12113&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300769
  Stats: 27 lines in 3 files changed: 1 ins; 20 del; 6 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