RFR: Report illegal transitions verbosely, and remove some no-op transitions

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/region-transition-asserts/webrev.01/

When illegal transition is detected, the failure should mention what region had failed to transit.
The fix also needs to rename some transition methods and remove their no-op reflexive transitions,
for consistency.

Testing: hotspot_gc_shenandoah (fastdebug)

Thanks,
-Aleksey