RFR: JDK-8319784: VM crash during heap dump after JDK-8287061 [v2]

[Cesar Soares Lucas]

> This fixes an incorrect assert in `debugInfo.hpp`. The assert is incorrect because this method may be called before rematerialization of the ObjectValue takes place, and therefore, `_selected` will be nullptr. This pull request patches the method to just return `Handle() => nullptr` if the rematerialization of the object hasn't taken place yet. This is the same behavior used by scalar-replaced objects not participating in merges.
> 
> I added a test to reproduce a scenario where the assert was causing a problem.
> 
> Tested on Ubuntu/MacOS/Windows x86_64 tier1-4.

Cesar Soares Lucas has updated the pull request incrementally with one additional commit since the last revision:

  Address PR feedback: improve test to not let lingering heapdump files.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16622/files
  - new: https://git.openjdk.org/jdk/pull/16622/files/aa723f50..26f7c936

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16622&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16622&range=00-01

  Stats: 51 lines in 2 files changed: 42 ins; 6 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16622.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16622/head:pull/16622

PR: https://git.openjdk.org/jdk/pull/16622