RFR: 8215118: Reduce HeapRegion to only have one member for index in CSet [v2]

[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:

  after kstefanj discussion

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5122/files
  - new: https://git.openjdk.java.net/jdk/pull/5122/files/e1d5d026..43cda20b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5122&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5122&range=00-01

  Stats: 56 lines in 8 files changed: 8 ins; 28 del; 20 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