RFR: JDK-8321565: [REDO] Heap dump does not contain virtual Thread stack references

[Alex Menkov]

Original fix for JDK-8299426 (Heap dump does not contain virtual Thread stack references, #16665) caused failures of new test (added while #16665 was under review):
test/hotspot/jtreg/compiler/c2/TestReduceAllocationAndHeapDump.java in many tears and was reverted.

Segmented heap dump assumes "merge" stage is executed outside of safepoint (to not block the VM), but heap dump may happen during safepoint (and TestReduceAllocationAndHeapDump.java test provoke the case).
The change contains original fix for JDK-8299426 ("[original fix](https://github.com/openjdk/jdk/commit/bdbf768eafa86e0007aca4188e0567693afe9071)") and removes asserts from HeapMerger ([allow heapdump in safepoints](https://github.com/openjdk/jdk/commit/44670ca4bf55dd2a5f1f44686758844aed68937e)).

Run tier1-3 and heapdump-related tests.

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Commit messages:
 - allow heapdump in safepoints
 - original fix

Changes: https://git.openjdk.org/jdk/pull/17040/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17040&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321565
  Stats: 319 lines in 3 files changed: 152 ins; 81 del; 86 mod
  Patch: https://git.openjdk.org/jdk/pull/17040.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17040/head:pull/17040

PR: https://git.openjdk.org/jdk/pull/17040