[jdk17u-dev] RFR: 8278146: G1: Rework VM_G1Concurrent VMOp to clearly identify it as pause

[Aleksey Shipilev]

Clean backport to improve detailed VM diagnostics.

@tschatzl, are you good with this in 17u?

Additional testing:
 - [x] Linux x86_64 fastdebug `tier1`

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Commit messages:
 - Backport 3a1fca3adf3111a966cb62d926b95acc89b7fe97

Changes: https://git.openjdk.org/jdk17u-dev/pull/910/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=910&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8278146
  Stats: 67 lines in 4 files changed: 29 ins; 24 del; 14 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/910.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/910/head:pull/910

PR: https://git.openjdk.org/jdk17u-dev/pull/910