RFR: 8329529: Serial: Move _saved_mark_word out of ContiguousSpace [v3]

[Guoxiong Li]

> Hi all,
> 
> This patch removes the field `ContiguousSpace::_saved_mark_word` and its related methods, adds the corresponding fields to `DefNewGeneration` and `TenuredGeneration`, and moves the method `ContiguousSpace::oop_since_save_marks_iterate` to `Generation`. Many usages are adjusted as well.
> 
> An alternative way is add a class which extends the `ContiguousSpace` to place `_saved_mark_word` and its related methods. But this way would increase the class hierarchy. Not really know whether it deserves to do that.
> 
> The tests `make test-tier1_gc` passed locally. Thanks for taking the time to review.
> 
> Best Regards,
> -- Guoxiong

Guoxiong Li has updated the pull request incrementally with one additional commit since the last revision:

  Remove unncessary comments.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18603/files
  - new: https://git.openjdk.org/jdk/pull/18603/files/8ef588b5..af974a14

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18603&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18603&range=01-02

  Stats: 2 lines in 2 files changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18603.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18603/head:pull/18603

PR: https://git.openjdk.org/jdk/pull/18603