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

[William Kemper]

* 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.

-------------

Commit messages:
 - Use tags to control log volume, rename methods in collection set, general cleanup

Changes: https://git.openjdk.org/jdk/pull/27963/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27963&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370521
  Stats: 75 lines in 12 files changed: 18 ins; 10 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/27963.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27963/head:pull/27963

PR: https://git.openjdk.org/jdk/pull/27963