RFR: 8261413: Shenandoah: Disable class-unloading in I-U mode

[Roman Kennke]

JDK-8261341 describes a serious problem with I-U mode and class-unloading. Let's disable class-unloading in I-U for now as a workaround.

Testing:
 - [ ] hotspot_gc_shenandoah
 - [ ] tier1 (+UseShenandoahGC +IU)
 - [x] runtime/CreateMirror/ArraysNewInstanceBug.java (+UseShenandoahGC +IU +aggressive) many times in a row w/o failure

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Commit messages:
 - 8261413: Shenandoah: Disable class-unloading in I-U mode

Changes: https://git.openjdk.java.net/jdk/pull/2477/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2477&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261413
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2477.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2477/head:pull/2477

PR: https://git.openjdk.java.net/jdk/pull/2477