RFR: Fix budgeting assertion

[Kelvin Nilsen]

On Fri, 5 Aug 2022 20:54:35 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> 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.

There's another assertion failure related to evacuating budgeting that has turned up.  After investigating this issue, I want to make one other change to this code before we merge it.

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