RFR: 8290525: Move HeapRegion::_compaction_top to G1FullCollector [v2]

[Thomas Schatzl]

> Hi all,
> 
>   can I have reviews for this change that moves `HeapRegion::_compaction_top` into the `G1FullCollector` class; my reasoning is that these values are transient temporary values only ever useful for the full collection algorithm, basically results of the application of the algorithm, that are otherwise unused. This makes `HeapRegion` a bit easier to understand/read.
> 
> Testing: tier1-3
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  kbarrett review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9560/files
  - new: https://git.openjdk.org/jdk/pull/9560/files/16d264f6..57d69c14

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9560&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9560&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9560.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9560/head:pull/9560

PR: https://git.openjdk.org/jdk/pull/9560