RFR: Assert Shenandoah-specific safepoints instead of generic ones

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/assert-shenandoah-sps/webrev.01/

There are intermittent assertion failures in ShenandoahHeap::assert_gc_workers. It happens almost
always when "Concurrent cleanup" is running in the middle of biased locking revocation safepoint.
Then, assertion code thinks it should run with UseParallelGC threads.

Fixed that by checking for Shenandoah-specific safepoints only. Other asserts are changed to verify
all Shenandoah-specific safepoints are covered.

Testing: hotspot_gc_shenandoah (fastdebug)

Thanks,
-Aleksey