RFR: 8344985: GenShen: Refactor arraycopy barrier for generational mode

[William Kemper]

Make the difference in behavior for generational and other modes more clear.

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Commit messages:
 - Make the separation between generational and other modes more clear

Changes: https://git.openjdk.org/shenandoah/pull/553/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=553&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344985
  Stats: 14 lines in 1 file changed: 6 ins; 7 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/553.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/553/head:pull/553

PR: https://git.openjdk.org/shenandoah/pull/553