RFR: 8309953: Strengthen and optimize oopDesc age methods [v2]

[Aleksey Shipilev]

> See the RFE for discussion. Basically, there is little reason to do two loads of mark word, when we can do one. 
> 
> Additional testing:
>  - [x] Eyeballing generated code
>  - [x] Linux x86_64 fastdebug `tier1 tier2 tier3`
>  - [x] Linux AArch64 fastdebug `tier1 tier2 tier3`

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into JDK-8309953-oopdesc-age
 - Revert helper method removals
 - Fix

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/14456/files
  - new: https://git.openjdk.org/jdk/pull/14456/files/cfc85cf5..7ad3c72d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14456&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14456&range=00-01

  Stats: 68073 lines in 1080 files changed: 50776 ins; 13698 del; 3599 mod
  Patch: https://git.openjdk.org/jdk/pull/14456.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14456/head:pull/14456

PR: https://git.openjdk.org/jdk/pull/14456