RFR: 8258244: Shenandoah: Not expecting forwarded object in roots during mark after JDK-8240868

[Zhengyu Gu]

This is a cleanup, no forwarded objects are expected in roots during mark phase after JDK-8240868.

There may be forwarded objects during full gc marking, if it is upgraded from degenerated GC, but roots are fixed before full gc marking happens.

- [x] hotspot_gc_shenandoah

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Commit messages:
 - Merge branch 'master' into JDK-8258244-no-forwarded-mark
 - JDK-8258244
 - JDK-8258239

Changes: https://git.openjdk.java.net/jdk/pull/1772/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1772&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258244
  Stats: 20 lines in 1 file changed: 0 ins; 14 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1772.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1772/head:pull/1772

PR: https://git.openjdk.java.net/jdk/pull/1772