RFR: 8366063: Parallel: Refactor copy_unmarked_to_survivor_space

[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.

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Commit messages:
 - pgc-young-gc-alloc-refactor

Changes: https://git.openjdk.org/jdk/pull/26922/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26922&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366063
  Stats: 149 lines in 2 files changed: 81 ins; 62 del; 6 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