[14] RFR (S) 8243487: Shenandoah: make _num_phases illegal phase type

[Aleksey Shipilev]

Original RFE:
  https://bugs.openjdk.java.net/browse/JDK-8243487
  https://hg.openjdk.java.net/jdk/jdk/rev/93e446b95cfc

This is a part of bulk backport of enhanced Shenandoah logging to 14u. The context is a bit
different, because the root list in assert is a bit different.

14u webrev:
  https://cr.openjdk.java.net/~shade/8243487/webrev.14u.01/

Testing: hotspot_gc_shenandoah, eyeballing gc logs

-- 
Thanks,
-Aleksey