RFR: 8366063: Parallel: Refactor copy_unmarked_to_survivor_space [v2]

[Albert Mingkun Yang]

> Extract out young/old generation allocation logic so that the evacuation method flow becomes clear and easier to follow.
> 
> Test: tier1-5; perf neutral for specjvm2008, dacapo and some synthetic bms.

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 pgc-young-gc-alloc-refactor
 - pgc-young-gc-alloc-refactor

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26922/files
  - new: https://git.openjdk.org/jdk/pull/26922/files/4127e40d..ec6f3b8f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26922&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26922&range=00-01

  Stats: 17491 lines in 832 files changed: 13013 ins; 1815 del; 2663 mod
  Patch: https://git.openjdk.org/jdk/pull/26922.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26922/head:pull/26922

PR: https://git.openjdk.org/jdk/pull/26922