RFR: JDK-8315479: GenShen: Expand old-gen while selecting collection set during GLOBAL GC [v4]

[Kelvin Nilsen]

> With GenShen, the size of old generation is as small as possible in order to maximize memory available to young gen.  This generally allows less frequent (and therefore more efficient) young-gen collections.
> 
> Under normal operation (concurrent marking of old followed by a series of mixed evacuations), old-gen is expanded at the end of a previous GC cycle in order to accommodate evacuation of mixed-evacuation candidates.
> 
> However, when we do a GLOBAL collection, we need to expand OLD gen during the selection of the collection set, depending on how many old-gen regions are selected to be evacuated during the GC evacuation phase.  This PR makes the change.

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

  Fixup comment

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/318/files
  - new: https://git.openjdk.org/shenandoah/pull/318/files/10f8e145..867fcf07

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=318&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=318&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/318.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/318/head:pull/318

PR: https://git.openjdk.org/shenandoah/pull/318