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

[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

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Commit messages:
 - preclean

Changes: https://git.openjdk.java.net/jdk/pull/6327/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6327&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276887
  Stats: 189 lines in 6 files changed: 38 ins; 128 del; 23 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