RFR: 8276887: G1: Move precleaning to Concurrent Mark From Roots subphase [v4]

[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:

  review

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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/f73a6db5..4c3e7514

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6327&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6327&range=02-03

  Stats: 8 lines in 2 files changed: 2 ins; 0 del; 6 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