RFR: 8364628: Serial: Refactor SerialHeap::mem_allocate_work [v2]

[Albert Mingkun Yang]

> Extract out old-gen allocation outside the critical-region to streamline the flow in `mem_allocate_work`.
> 
> Test: tier1-5

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 two additional commits since the last revision:

 - Merge branch 'master' into sgc-mem-allocate
 - sgc-mem-allocate

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26619/files
  - new: https://git.openjdk.org/jdk/pull/26619/files/e1c6c3de..0b0587ad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26619&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26619&range=00-01

  Stats: 15548 lines in 418 files changed: 8300 ins; 5464 del; 1784 mod
  Patch: https://git.openjdk.org/jdk/pull/26619.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26619/head:pull/26619

PR: https://git.openjdk.org/jdk/pull/26619