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

[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.

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Commit messages:
 - Allow old-gen to expand when mutator memory is available

Changes: https://git.openjdk.org/shenandoah/pull/394/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=394&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325670
  Stats: 7 lines in 1 file changed: 4 ins; 3 del; 0 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