RFR: 8332548: GenShen: Factor generational mode out of gc helpers

[William Kemper]

Some of the generational mode support in `shenandoahConcurrentGC.cpp` and `shenandoahDegeneratedGC.cpp` can be factored into generational mode specific files and coalesced under fewer generational mode checks.

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Commit messages:
 - Cleanup IDE artifacts
 - Move some methods into shGenerationalHeap
 - Reduce generational mode checks

Changes: https://git.openjdk.org/shenandoah/pull/436/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=436&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332548
  Stats: 323 lines in 6 files changed: 124 ins; 157 del; 42 mod
  Patch: https://git.openjdk.org/shenandoah/pull/436.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/436/head:pull/436

PR: https://git.openjdk.org/shenandoah/pull/436