RFR: 8330006: Serial: Extract out ContiguousSpace::block_start_const [v4]

[Guoxiong Li]

> Hi all,
> 
> This patch moves the method `ContiguousSpace::block_start_const` to `DefNewGeneration` (note: not the parent class `Generation`) and moves the method `TenuredSpace::block_start_const` to `TenuredGeneration`. These two methods don't need  the same signature so I don't add a virtual method to the parent class `Generation`.
> 
> 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:

  Inline

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18722/files
  - new: https://git.openjdk.org/jdk/pull/18722/files/4181badc..d2ca1c04

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18722&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18722&range=02-03

  Stats: 8 lines in 2 files changed: 0 ins; 6 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18722.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18722/head:pull/18722

PR: https://git.openjdk.org/jdk/pull/18722