RFR: 8215118: Reduce HeapRegion to only have one member for index in CSet

[Ivan Walulya]

Hi all,

Please review this change to maintain only a single member variable to indicate the region's index in a collection set: either the actual collection set for young regions or the optional regions array if the region is considered optional during a mixed collection.

Testing: Tier 1-7

-------------

Commit messages:
 - 8215118: Reduce HeapRegion to only have one member for index in CSet

Changes: https://git.openjdk.java.net/jdk/pull/5122/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5122&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8215118
  Stats: 42 lines in 4 files changed: 25 ins; 3 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5122.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5122/head:pull/5122

PR: https://git.openjdk.java.net/jdk/pull/5122