RFR: 8264026: Remove dependency between free collection set and eagerly reclaim humongous object tasks

[Thomas Schatzl]

Hi,

  I am currently working on [JDK-8214237](https://bugs.openjdk.java.net/browse/JDK-8214237) to merge some phases from `G1CollectedHeap::post_evacuate_collection_set()` into less parallel phases.

While investigating dependencies I found that `free_collection_set` and `eagerly_reclaim_humongous_regions` have a dependency: `free_collection_set` rebuilds the free list at the end (in another parallel phase :( ), and `eagerly_reclaim_humongous_regions` adds to those regions.

This dependency is unnecessary: just move the rebuilding of the free list after eagerly reclaiming humongous regions, and neither needs to care about updating the free regions list. 

Testing: hs-tier1-5

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Commit messages:
 - Remove residues from other change
 - Initial implementation

Changes: https://git.openjdk.java.net/jdk/pull/3154/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3154&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264026
  Stats: 17 lines in 2 files changed: 10 ins; 5 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3154.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3154/head:pull/3154

PR: https://git.openjdk.java.net/jdk/pull/3154