[jdk17u] RFR: 8269924: Shenandoah: Introduce weak/strong marking asserts

[Aleksey Shipilev]

Clean backport.

Additional testing:
 - [x] `hotspot_gc_shenandoah` is passing

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Commit messages:
 - Backport 515113d8589875e76980ebf7473cb81f778bb86f

Changes: https://git.openjdk.java.net/jdk17u/pull/26/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=26&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269924
  Stats: 46 lines in 2 files changed: 46 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/26.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/26/head:pull/26

PR: https://git.openjdk.java.net/jdk17u/pull/26