RFR: 8261973: Shenandoah: Cleanup/simplify root verifier

[Zhengyu Gu]

Root processing has gone through significant changes. For example, we used to mark through weak roots when class unloading is off, that is no long the case, OopStorages also simplify roots.

Shenandoah root verifier can be simplified into 2 cases, with/without class unloading.

- [x] hotspot_gc_shenandoah with -XX:+ShenandoahVerify

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Commit messages:
 - update
 - 8261973

Changes: https://git.openjdk.java.net/jdk/pull/2643/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2643&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261973
  Stats: 198 lines in 4 files changed: 10 ins; 159 del; 29 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2643.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2643/head:pull/2643

PR: https://git.openjdk.java.net/jdk/pull/2643