RFR: 8370850: Shenandoah: Simplify collector allocation to save unnecessary region iteration [v2]

[Xiaolong Peng]

On Fri, 31 Oct 2025 20:54:47 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> I was confused about this part, if we can find a FREE region in the partition, why we still need to test if the generation must have free_unaffiliated_regions, I can add can_allocate_in_new_region back to retain the original behavior.
>
> The young and old collector reserves both have `FREE` regions, but we need to make sure we don't exceed the reserves for one or the other.

I have added the can_allocate_in_new_region check back, it should have same behavior now.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28036#discussion_r2482669646