RFR: 8314329: AgeTable: add is_clear() & allocation spec, and relax assert to allow use of 0-index slot [v3]

[Y. Srinivas Ramakrishna]

> I'd like to propose the following changes to the AgeTable (motivated by the uses in generational shenandoah):

Y. Srinivas Ramakrishna has updated the pull request incrementally with one additional commit since the last revision:

  Changes from @albertnetymk review : used #ifndef PRODUCT rather than
  PRODUCT_RETURN0 for is_clear().

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17470/files
  - new: https://git.openjdk.org/jdk/pull/17470/files/ede84ca2..3572babf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17470&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17470&range=01-02

  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17470.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17470/head:pull/17470

PR: https://git.openjdk.org/jdk/pull/17470