RFR: 8215118: Reduce HeapRegion to only have one member for index in CSet [v4]

[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

Ivan Walulya has updated the pull request incrementally with one additional commit since the last revision:

  error in branch merge

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5122/files
  - new: https://git.openjdk.java.net/jdk/pull/5122/files/eaaf5683..dde6a82c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5122&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5122&range=02-03

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 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