RFR: 8325670: GenShen: Allow old to expand at end of each GC [v2]

[Kelvin Nilsen]

> At the end of GC, we set aside collector reserves to satisfy anticipated needs of the next GC.
> 
> This PR reverts a change that accidentally prevents old-gen from being enlarged by this action.  The observed failure condition was that mixed evacuations were not able to be performed, because old-gen was not large enough to receive the results of the desired evacuations.

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

  Refine calculation of max_old_reserve

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/394/files
  - new: https://git.openjdk.org/shenandoah/pull/394/files/f583f713..e0761e82

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=394&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=394&range=00-01

  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/394.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/394/head:pull/394

PR: https://git.openjdk.org/shenandoah/pull/394