[vm] Fix race in vm/cc/SafepointOperation_SafepointOpBlockedWhileDeoptSafepointOp.

LongDeoptTask: MarkAndNotify(kFinishedDeoptOperation)
         main: MarkAndNotify(kPleaseExit)

If main wins, LongDeoptTask will override the state and wait for a kPleaseExit that will never come.

TEST=ci
Change-Id: I2f29da2fca980ca8f2b3c42477c4e793d7379af4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/416381
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google

runtime/vm/heap/safepoint_test.cc

@@ -279,6 +279,7 @@ ISOLATE_UNIT_TEST_CASE(
     // progress.
     deopt->MarkAndNotify(LongDeoptTask::kFinishDeoptOperation);
     gc->WaitUntil(WaiterTask::kExitedSafepoint);
+    deopt->WaitUntil(LongDeoptTask::kFinishedDeoptOperation);
 
     // Make both threads exit the isolate group.
     deopt->MarkAndNotify(LongDeoptTask::kPleaseExit);