RFR: Replace risky SBS::need_update_refs_barrier with straightforward check

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/sbs-risky-check/webrev.01/

Heuristics has the automatic switch between separate-UR and CM-with-UR modes. Current check relies
on heuristics()->update_refs() to figure out if we should check ur-in-progress or mark-in-progress
when updating the references. This is risky, because heuristics may have stale idea about which mode
to operate in. It is safer to check all potential GC states at once.

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey