RFR: 8294712: G1: Use index-base iteration for G1FlushHumongousCandidateRemSets

[Albert Mingkun Yang]

Simple change of reducing the fast-path cost in `G1FlushHumongousCandidateRemSets`.

Test: hotspot_gc

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Commit messages:
 - g1-region-index

Changes: https://git.openjdk.org/jdk/pull/10531/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10531&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294712
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/10531.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10531/head:pull/10531

PR: https://git.openjdk.org/jdk/pull/10531