RFR: 8324325: [Genshen] Normalize wrt AgeTable changes from JDK-8314329

[Y. Srinivas Ramakrishna]

Some trivial changes stemming from upstream changes for JDK-8314329.

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Commit messages:
 - iconst is_clear() etc.
 - Merge branch 'master' into isclear
 - Early return upon from is_clear() as soon as we find a positive entry.
 - AgeTable::is_clear() now NOT_PRODUCT rather than PRODUCT_RETURN0. Straighten

Changes: https://git.openjdk.org/shenandoah/pull/386/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=386&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324325
  Stats: 13 lines in 4 files changed: 7 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/shenandoah/pull/386.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/386/head:pull/386

PR: https://git.openjdk.org/shenandoah/pull/386