RFR: 8344152: GenShen: Initialize RegionData in product builds

[William Kemper]

Hygiene improvement.

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Commit messages:
 - Initialize RegionData fields in product builds

Changes: https://git.openjdk.org/shenandoah/pull/537/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=537&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344152
  Stats: 8 lines in 2 files changed: 4 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/537.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/537/head:pull/537

PR: https://git.openjdk.org/shenandoah/pull/537