RFR: 8278146: G1: Rework VM_G1Concurrent VMOp to clearly identify it as pause [v3]

[Aleksey Shipilev]

> Our support engineers asked this:
> 
>> I see these G1Concurrent safepoints in JDK17:
>> [0.064s][info][safepoint ] Safepoint "G1Concurrent", Time since last: 1666947 ns, Reaching
> safepoint: 79150 ns, At safepoint: 349999 ns, Total: 429149 ns
>> I've always thought that "concurrent" and "safepoint" are basically antonyms.
>> What is a G1Concurrent safepoint? How can a concurrent event require a safepoint?
> 
> I agree that's confusing. This patch splits the VM_G1Concurrent into two exactly named VMOp-s, so that we get:
> 
> 
> [6.527s][info][gc       ] GC(7) Pause Remark 64M->64M(224M) 218.847ms
> [6.527s][info][safepoint] Safepoint "G1PauseRemark", Time since last: 17493991 ns, Reaching safepoint: 506830 ns, At safepoint: 218950374 ns, Total: 219457204 ns
> [6.536s][info][gc       ] GC(7) Pause Cleanup 71M->71M(224M) 0.177ms
> [6.536s][info][safepoint] Safepoint "G1PauseCleanup", Time since last: 8250157 ns, Reaching safepoint: 884967 ns, At safepoint: 223964 ns, Total: 1108931 ns
> [6.537s][info][gc       ] GC(7) Concurrent Mark Cycle 247.051ms
> 
> 
> Additional testing:
>  - [x] Linux x86_64 fastdebug `tier1`

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Use override
 - Merge branch 'master' into JDK-8278146-g1-concurrent-vmop
 - Review Thomas
 - Merge branch 'master' into JDK-8278146-g1-concurrent-vmop
 - Whitespace and touchups
 - Basic implementation

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6677/files
  - new: https://git.openjdk.java.net/jdk/pull/6677/files/06479f45..e26df883

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6677&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6677&range=01-02

  Stats: 30248 lines in 906 files changed: 20806 ins; 5250 del; 4192 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6677.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6677/head:pull/6677

PR: https://git.openjdk.java.net/jdk/pull/6677