RFR: 8377396: GenShen: Consolidate and simplify in place region promotions [v3]

[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.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Add new line

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29618/files
  - new: https://git.openjdk.org/jdk/pull/29618/files/9872788d..c1cf97ad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29618&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29618&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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