RFR: 8293252: Shenandoah: ThreadMXBean synchronizer tests crash with IU+aggressive mode

[Ashutosh Mehra]

Please review the fix for the assertion failure when running ThreadMXBean synchronizer code in fastdebug mode.

Please note that the assertion failure happens in regular (satb) mode as well, not just in iu mode.

At the point of assertion failure, the JVM is at a safepoint as the VMThread is executing VM_ThreadDump operation and the GC worker threads are paused after the mark phase.
Assertion happened when the VMThread performs `NativeAccess<>::oop_store()` (as part of creating an `OopHandle`) on an object which has been marked and is in the collection set. But the failing assertion in `oop_store_not_in_heap()` expects the oop is not in the collection set.

The assertion is conditional on the statement `value != NULL && !ShenandoahHeap::heap()->cancelled_gc()`

	shenandoah_assert_not_in_cset_if(addr, value, value != NULL && !ShenandoahHeap::heap()->cancelled_gc());

But it looks like it is missing another important condition - the GC should be in marking phase.
i.e. the assertion is valid only if it is protected by `ShenandoahHeap::heap()->is_concurrent_mark_in_progress()`.
So the correct assertion should be:

	shenandoah_assert_not_in_cset_if(addr, value, value != NULL && !ShenandoahHeap::heap()->cancelled_gc() && ShenandoahHeap::heap()->is_concurrent_mark_in_progress());

In fact this assertion is already present in one of its caller (`oop_store_in_heap()1) in its negative form:

	shenandoah_assert_not_in_cset_except    (addr, value, value == NULL || ShenandoahHeap::heap()->cancelled_gc() || !ShenandoahHeap::heap()->is_concurrent_mark_in_progress());


Also, it reads strange that `oop_store_in_heap()` would call `oop_store_not_in_heap()`.
So I have moved the code to a separate method `oop_store_common()` that gets called by both `oop_store_in_heap()` and `oop_store_not_in_heap()`.

Tested it by running following tests in fastdebug mode:

- hotspot_gc_shenandoah
- java/lang/management/ThreadMXBean/MyOwnSynchronizer.java (50 times with -XX:ShenandoahGCHeuristics=aggressive)
- java/lang/management/ThreadMXBean/LockedSynchronizers.java (50 times with -XX:ShenandoahGCHeuristics=aggressive)

Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>

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Commit messages:
 - 8293252: Shenandoah: ThreadMXBean synchronizer tests crash with IU+aggressive mode

Changes: https://git.openjdk.org/jdk/pull/10268/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10268&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293252
  Stats: 12 lines in 2 files changed: 8 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10268.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10268/head:pull/10268

PR: https://git.openjdk.org/jdk/pull/10268