RFR: 8366063: Parallel: Refactor copy_unmarked_to_survivor_space

[Thomas Schatzl]

On Mon, 25 Aug 2025 08:52:05 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> 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.

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/26922#pullrequestreview-3154523400