RFR: 8270009: Factor out and shuffle methods in G1CollectedHeap::do_collection_pause_at_safepoint_helper

[Thomas Schatzl]

Hi all,

  can I get reviews for this change that factors out a few methods from `do_collection_pause_at_safepoint_helper` and reshuffles code from `gc_prologue`/`gc_epilogue` to be able to put those into parallel phase(s) later.

Testing: tier1-5

Thanks,
  Thomas

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Commit messages:
 - Initial implementation of method reshuffling

Changes: https://git.openjdk.java.net/jdk/pull/4744/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4744&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270009
  Stats: 212 lines in 3 files changed: 106 ins; 89 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4744.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4744/head:pull/4744

PR: https://git.openjdk.java.net/jdk/pull/4744