RFR: 8377396: GenShen: Consolidate and simplify in place region promotions

[William Kemper]

The logic to select and execute in place region promotions is spread out across different files. This is a mechanical refactoring to simplify the code and encapsulate the abstraction.

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Commit messages:
 - Fix missingforward declaration and another include sort order
 - Add missing include
 - Document public methods
 - Colocate code for planning and executing for promoting regions in place

Changes: https://git.openjdk.org/jdk/pull/29618/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29618&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377396
  Stats: 701 lines in 6 files changed: 406 ins; 279 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/29618.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29618/head:pull/29618

PR: https://git.openjdk.org/jdk/pull/29618