RFR: 8276887: G1: Move precleaning to Concurrent Mark From Roots subphase [v3]

[Albert Mingkun Yang]

> Simple change of moving the "Preclean" subphase into "Mark from roots" subphase so that precleaning becomes parallel automatically. Evaluation using a contrived java program shows that multiple GC threads do precleaning. More detailed testing results are available in the JBS ticket.
> 
> Test: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with two additional commits since the last revision:

 - review
 - Revert "worker_id"
   
   This reverts commit 0841db5c5c2b39372058b6ba0d8027aae3716669.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6327/files
  - new: https://git.openjdk.java.net/jdk/pull/6327/files/0841db5c..f73a6db5

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6327&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6327&range=01-02

  Stats: 33 lines in 3 files changed: 16 ins; 5 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6327.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6327/head:pull/6327

PR: https://git.openjdk.java.net/jdk/pull/6327