RFR: 8361055: Serial: Inline SerialHeap::process_roots [v2]

[Albert Mingkun Yang]

> Refactor roots processing in Serial (young-gc and full-gc) to clean up the control-flow and make is clearer what roots and closures are used in each context.
> 
> Test: tier1-8

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26038/files
  - new: https://git.openjdk.org/jdk/pull/26038/files/6088695a..0743d5e5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26038&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26038&range=00-01

  Stats: 6 lines in 3 files changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/26038.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26038/head:pull/26038

PR: https://git.openjdk.org/jdk/pull/26038