RFR: 8332824: GenShen: Temporarily revert recent changes to GC helpers

[William Kemper]

This reverts commit 8f808276edfb163c14223f642f12dc6faddc49c6. This commit is breaking in stress test pipelines. I'm backing it out while it is debugged.

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Commit messages:
 - Revert "8332548: GenShen: Factor generational mode out of gc helpers"

Changes: https://git.openjdk.org/shenandoah/pull/437/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=437&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332824
  Stats: 342 lines in 6 files changed: 158 ins; 126 del; 58 mod
  Patch: https://git.openjdk.org/shenandoah/pull/437.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/437/head:pull/437

PR: https://git.openjdk.org/shenandoah/pull/437