8370521: GenShen: Various code cleanup related to promotion

[earthling-amzn]

• 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 #27632 to facilitate its review.

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Progress

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Issue

• JDK-8370521: GenShen: Various code cleanup related to promotion (Enhancement - P4)

Reviewers

• Francesco Andreuzzi (@fandreuz - Author)
• Kelvin Nilsen (@kdnilsen - Committer)

Reviewing

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