RFR: 8256265: G1: Improve parallelism in regions that failed evacuation [v6]

[Thomas Schatzl]

> Hi all,
> 
>   can I have reviews on this change that makes the evacuation failure remove forwards phase split the work across threads within regions?
> 
> This work is a continuation of PR#7047 from @Hamlin-Li and latest improvements from @albertnetymk in that thread. This is reflected in the first commit, I added a fair amount of changes to fix bugs and streamline the code.  
> 
> Testing: tier1-8 with G1EvacuationFailureALot enabled, tier1-5 as is

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

  sjohanss, ayang review 4

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9980/files
  - new: https://git.openjdk.org/jdk/pull/9980/files/3363fca0..10082a13

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9980&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9980&range=04-05

  Stats: 15 lines in 7 files changed: 2 ins; 1 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/9980.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9980/head:pull/9980

PR: https://git.openjdk.org/jdk/pull/9980