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

[Y. Srinivas Ramakrishna]

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

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Commit messages:
 - More consts and a bit more encapsulation of preselected regions.
 - Refactor to make interface narrow since state is implicitly managed in
 - Replace automatic array variables with resource area allocated arrays
 - Replace alloca() with automatic array allocation. Might be better to
 - Structured maintenance of preselected regions field: first iteration
 - Some more documentation comments.
 - Add/expand some documentation comments; remove some unused "tracking
 - Remove unnecessary `num_regions` parameter to method
 - Merge branch 'master' into couple_sizing_tenuring
 - jcheck  "trailing whitespace" clean.
 - ... and 3 more: https://git.openjdk.org/shenandoah/compare/dd011516...c49419b2

Changes: https://git.openjdk.org/shenandoah/pull/381/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=381&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/pull/381.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/381/head:pull/381

PR: https://git.openjdk.org/shenandoah/pull/381