RFR: Fix budgeting assertion [v3]

[Kelvin Nilsen]

> An assertion failure revealed problems with evacuation budgeting computations.  Three changes are reflected in this pull request:
> 
> 1. Memory set aside for the promotion reserve was not excluded from the allocation supplement
> 2. When the old evacuation reserve is larger than the memory consumed by old evacuation collection set, we loan the excess memory to allocation supplement to make sure this memory is not consumed by promotions.  The number of regions set aside from this reserve for the allocation supplement must be rounded down from the total excess memory.  (Before this patch, it was rounded up.)
> 3. The assertion test that was failing needed a <= comparison instead of a < comparison.

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

  Minor change and comments for reviewer

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/156/files
  - new: https://git.openjdk.org/shenandoah/pull/156/files/7d256dba..00504682

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=156&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=156&range=01-02

  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/156.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/156/head:pull/156

PR: https://git.openjdk.org/shenandoah/pull/156