RFR: 8343226: Shenandoah: Move tenuring threshold constraint functions into shared constraints

[William Kemper]

This change anticipates the [generational mode for Shenandoah](https://github.com/openjdk/jdk/pull/21273), which makes use of the `InitialTenuringThreshold` and `MaxTenuringThreshold` flags.

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Commit messages:
 - Move tenuring flag contraints, make them available to Shenandoah

Changes: https://git.openjdk.org/shenandoah/pull/529/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=529&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343226
  Stats: 115 lines in 4 files changed: 6 ins; 100 del; 9 mod
  Patch: https://git.openjdk.org/shenandoah/pull/529.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/529/head:pull/529

PR: https://git.openjdk.org/shenandoah/pull/529