RFR: Refactor budgeting to make the logic cleaner [v2]

[Kelvin Nilsen]

> This commit consolidates computations related to budgeting of reserves for old-gen evacuation and old-gen promotion within shenandoahGeneration::prepare_regions_and_collection_set().  Previously, these computations had been scattered between several functional units, including shenandoahHeuristics::choose_collection_set() and shenandoahOldHeuristics::prime_collection_set().  A previous pull request brought some of the redundant functionality into prepare_regions_and_collection_set().  This pull request removes the redundant computations from the other locations.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Isolate the code that reserves evacuation budgets into functions

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/148/files
  - new: https://git.openjdk.org/shenandoah/pull/148/files/94d5e73e..be1fb125

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=148&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=148&range=00-01

  Stats: 697 lines in 2 files changed: 358 ins; 336 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/148.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/148/head:pull/148

PR: https://git.openjdk.org/shenandoah/pull/148