RFR: 8270009: Factor out and shuffle methods in G1CollectedHeap::do_collection_pause_at_safepoint_helper [v3]

[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

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  Move back wait_for_root_region_scan

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4744/files
  - new: https://git.openjdk.java.net/jdk/pull/4744/files/b8fd8e09..999d66dd

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4744&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4744&range=01-02

  Stats: 7 lines in 1 file changed: 5 ins; 2 del; 0 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