RFR: 8323083: [GenShen] Alloca avoidance, const-safety, interface decluttering in promotion budgeting

[William Kemper]

8323083: [GenShen] Alloca avoidance, const-safety, interface decluttering in promotion budgeting

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Commit messages:
 - Backport 6cd8d04f283b1d0e65f40f816ccb10b9be59717c

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/11/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=11&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323083
  Stats: 164 lines in 6 files changed: 91 ins; 26 del; 47 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/11.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/11/head:pull/11

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/11