[14] RFR: 8246100: Shenandoah: walk roots in more efficient order

[Aleksey Shipilev]

Original RFE:
  https://bugs.openjdk.java.net/browse/JDK-8246100
  https://cr.openjdk.java.net/~shade/8246100/webrev.14u.01/

The context in 14u is a bit different. Re-did the patch, by shuffling the lines in the existing
files. 14u webrev:
  https://cr.openjdk.java.net/~shade/8246100/webrev.14u.01/

Testing: hotspot_gc_shenandoah, tier{1,2} with Shenandoah enabled

-- 
Thanks,
-Aleksey