RFR: 8392760: Improve liveness/remembered set verification for G1

[Thomas Schatzl]

Hi all,

  can I have reviews for this refactoring/cleanup of the liveness/remembered set verification code for G1?

Here is an overview of the changes
 - previously liveness and remembered set verification could be done separately. Thismade the code a bit more complicated than necessary; now that remembered set verification always follows liveness verification, the code can assume that and do some minor cleanup. I kept the remembered verification a separate closure (`VerifyRemSetClosure`) because the additional boiler plate code is negligible compared to the other duplication that would have been needed around failure count tracking.
 - factored out liveness and remembered set verification from `HeapRegion::verify`, and heavily cleaned up the resulting `HeapRegion::verify_liveness_and_remembered_set`. This includes some removal of code duplication wrt to the failure count tracking.
- factored out a special `G1VerificationClosure::is_oop_safe` method that adds more checking of the `Klass`; I hesitated to put this into `Universe::is_oop` as this is much more extensive checking (and I wanted implicit error reporting :))
- CollectedHeap::is_oop() should use `klass_raw` because otherwise it will immediately assert on oops with garbage Klass values (the new code will just return `false` as expected). 
- some special `oopDesc::print_name_on()` which does not crash on bad oops, similar to existing related debugging helpers.

Thanks,
  Thomas

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Commit messages:
 - initial attempts, nothing done

Changes: https://git.openjdk.org/jdk/pull/12660/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12660&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8392760
  Stats: 264 lines in 9 files changed: 108 ins; 86 del; 70 mod
  Patch: https://git.openjdk.org/jdk/pull/12660.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12660/head:pull/12660

PR: https://git.openjdk.org/jdk/pull/12660