RFR: 8261413: Shenandoah: Disable class-unloading in I-U mode [v2]

[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

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Some comment and output changes as requested by Aleksey

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2477/files
  - new: https://git.openjdk.java.net/jdk/pull/2477/files/491c41c1..e3c1b459

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2477&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2477&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 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