RFR: 8280932: G1: Rename HeapRegionRemSet::_code_roots accessors [v2]

[Albert Mingkun Yang]

> Mostly mechanical changes of renaming some methods/comments.
> 
> Test: build

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.java.net/jdk/pull/7285/files
  - new: https://git.openjdk.java.net/jdk/pull/7285/files/6a226cda..7473ca9b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7285&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7285&range=00-01

  Stats: 13 lines in 6 files changed: 0 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7285.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7285/head:pull/7285

PR: https://git.openjdk.java.net/jdk/pull/7285