RFR: 8342564: GenShen: Only reference young/old generation names in generational mode

[William Kemper]

When printing the heap, only reference young/old marking when running in generational mode.

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Commit messages:
 - Only reference young/old marking in generational mode

Changes: https://git.openjdk.org/shenandoah/pull/518/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=518&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342564
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/518.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/518/head:pull/518

PR: https://git.openjdk.org/shenandoah/pull/518