RFR: 8361055: Serial: Inline SerialHeap::process_roots [v3]

[Albert Mingkun Yang]

> Refactor roots processing in Serial (young-gc and full-gc) to clean up the control-flow and make is clearer what roots and closures are used in each context.
> 
> Test: tier1-8

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 four additional commits since the last revision:

 - review
 - Merge branch 'master' into sgc-roots
 - review
 - sgc-roots

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26038/files
  - new: https://git.openjdk.org/jdk/pull/26038/files/0743d5e5..8b779888

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26038&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26038&range=01-02

  Stats: 6323 lines in 255 files changed: 4224 ins; 1221 del; 878 mod
  Patch: https://git.openjdk.org/jdk/pull/26038.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26038/head:pull/26038

PR: https://git.openjdk.org/jdk/pull/26038