RFR: Fix assertion error with advance promotion budgeting [v2]

[Kelvin Nilsen]

> Round-off errors were resulting in an assertion error.  Budgeting calculations are "complicated" because only regions that are fully empty may be loaned from old-gen to young-gen.  This change recalculates certain values during budgeting adjustments that follow collection set selection rather than endeavoring to make changes to the values computed before collection set selection.  The API is simpler as a result.

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

  Allow round-off errors to impact assert

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/165/files
  - new: https://git.openjdk.org/shenandoah/pull/165/files/55eb39a4..e9430135

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=165&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=165&range=00-01

  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/165.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/165/head:pull/165

PR: https://git.openjdk.org/shenandoah/pull/165