RFR: 8333750: GenShen: Only instantiate young/old generations in generational mode [v2]

[William Kemper]

> Other modes should not instantiate or touch young/old generations.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Fix zero build

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/444/files
  - new: https://git.openjdk.org/shenandoah/pull/444/files/5e744806..b28513f7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=444&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=444&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/444.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/444/head:pull/444

PR: https://git.openjdk.org/shenandoah/pull/444