RFR: 8332082: Shenandoah: Use consistent tests to determine when pre-write barrier is active [v2]

[William Kemper]

> This is consistent with c1 and other platforms.

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Make all the barriers and verifiers use gc_state
   
   Still need to fix verifier for c2
 - Revert "Check for satb active flag, rather than gc state"
   
   This reverts commit 2769c97cbf5313c5c0f1336060ec39cb66584e3c.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19180/files
  - new: https://git.openjdk.org/jdk/pull/19180/files/2769c97c..3271fecf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19180&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19180&range=00-01

  Stats: 73 lines in 7 files changed: 17 ins; 31 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/19180.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19180/head:pull/19180

PR: https://git.openjdk.org/jdk/pull/19180