RFR: 8370521: GenShen: Various code cleanup related to promotion

[Kelvin Nilsen]

On Thu, 23 Oct 2025 22:50:04 GMT, William Kemper <wkemper at openjdk.org> wrote:

> * Use more log tags for finer grained control over debug output. 
> * Add instrumentation for plab and cset decisions
> * Fix typos in comments and minor formatting issues
> * Rename some methods and variables for easier reading
> 
> There are no behavioral changes here. These changes were hoisted from https://github.com/openjdk/jdk/pull/27632 to facilitate its review.

Marked as reviewed by kdnilsen (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/27963#pullrequestreview-3378773101