RFR: 8234446: Post-CMS workgroup hierarchy cleanup [v2]

[Albert Mingkun Yang]

> Collapse workgroup hierarchy and some general cleanup.
> 
> Tested; hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3663/files
  - new: https://git.openjdk.java.net/jdk/pull/3663/files/ea674346..a194f5ff

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3663&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3663&range=00-01

  Stats: 17 lines in 3 files changed: 0 ins; 1 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3663.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3663/head:pull/3663

PR: https://git.openjdk.java.net/jdk/pull/3663