RFR: Loan from old should align on region size

[Kelvin Nilsen]

This fixes an error detected by a recently added assertion on a different branch.  When loaning memory from old-gen to young-gen, it is important the total amount of the loan be an integral number of ShenandoahHeapRegions.  The total loan represents a sum of memory loaned to support young evacuations and memory loaned to support mutator allocations made while GC is active.  The sum of these two values is a multiple of region size.  The previous version of the code accidentally loaned only the memory dedicated to supporting mutator allocations, which is not necessarily a multiple of the region size.

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

Commit messages:
 - Merge remote-tracking branch 'GitFarmBranch/loan-from-old-should-align-on-region-size' into loan-from-old-should-align-on-region-size
 - Fix alignment error in old loans to young

Changes: https://git.openjdk.org/shenandoah/pull/213/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=213&range=00
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/213.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/213/head:pull/213

PR: https://git.openjdk.org/shenandoah/pull/213