RFR: Update roots should always handle derived pointers

[Aleksey Shipilev]

Hi,

Follow my logic here.

It looks to me that every root modification needs to be wrapped with DPT::clear
and DPT::update_pointers to capture the rare stack-alloced beasts. However, we
are missing this pair for Final Update Refs. Therefore, this is the valid
fix/cleanup:
  http://cr.openjdk.java.net/~shade/shenandoah/update-roots-dpt/webrev.01/

(Also checked all usages of ShenandoahRootEvacuator)

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey