RFR (XS) 8232205: Shenandoah: missing "Update References" -> "Update Roots" tracing

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8232205

Noticed that -Xlog:gc+stats does not print "Update Roots" section for "Update References". This is a
regression since JDK-8223951.

Fix:
  https://cr.openjdk.java.net/~shade/8232205/webrev.01/

Testing: hotspot_gc_shenandoah, eyeballing gc+stats

-- 
Thanks,
-Aleksey