RFR: 8330145: Serial: Refactor SerialHeap::scan_evacuated_objs

[Albert Mingkun Yang]

Simple refactoring to extract oop-scanning logic from generation.

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Commit messages:
 - s1-scan-evac-obj

Changes: https://git.openjdk.org/jdk/pull/18750/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18750&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330145
  Stats: 119 lines in 8 files changed: 28 ins; 88 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18750.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18750/head:pull/18750

PR: https://git.openjdk.org/jdk/pull/18750