RFR: 8236176: Parallel GC SplitInfo comment should be updated for shadow regions

[Ivan Walulya]

Hi all,

Please review this small change to add a high-level description of how shadow regions are utilized during the compact phase of the parallel GC. There is no special handling of Splitinfo by the shadow regions; the destination computation and the handling of partial_objects are done during the summary phase while the shadow regions optimization only applies to compaction. 

Additionally, some unused variables are cleaned up.

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Commit messages:
 - cleanup
 - minor type
 - Merge branch 'master' into JDK-8236176_Paralle_SplitInfo_shadow_regions
 - 8236176: Parallel GC SplitInfo comment should be updated for shadow regions

Changes: https://git.openjdk.java.net/jdk/pull/5195/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5195&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236176
  Stats: 30 lines in 2 files changed: 19 ins; 10 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5195.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5195/head:pull/5195

PR: https://git.openjdk.java.net/jdk/pull/5195