RFR: Use static assert to validate card offset encoding mask

[William Kemper]

Follow up to https://github.com/openjdk/shenandoah/pull/275 - replace runtime assert with static assert.

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Commit messages:
 - Use static assert to validate card offset encoding mask

Changes: https://git.openjdk.org/shenandoah/pull/279/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=279&range=00
  Stats: 6 lines in 1 file changed: 5 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/279.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/279/head:pull/279

PR: https://git.openjdk.org/shenandoah/pull/279