RFR: 8330071: GenShen: Allow old to expand again at end of each GC [v2]

[Kelvin Nilsen]

> This corrects errors in the implementations of max_size_for(OLD) and min_size_for(OLD).  These errors were preventing expansion of OLD in preparation for subsequent mixed-evacuation cycles.

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

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/421/files
  - new: https://git.openjdk.org/shenandoah/pull/421/files/d8813000..f23113f5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=421&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=421&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/421.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/421/head:pull/421

PR: https://git.openjdk.org/shenandoah/pull/421