RFR: 8276833: G1: make G1EvacFailureRegions::par_iterate const

[Hamlin Li]

This is a trivial code enhancement to make G1EvacFailureRegions::par_iterate const.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6302/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6302&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276833
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6302.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6302/head:pull/6302

PR: https://git.openjdk.java.net/jdk/pull/6302