RFR: 8276887: G1: Move precleaning to Concurrent Mark From Roots subphase [v2]

[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 one additional commit since the last revision:

  worker_id

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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/a0cd3dc0..0841db5c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6327&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6327&range=00-01

  Stats: 10 lines in 3 files changed: 3 ins; 2 del; 5 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