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

[Roman Kennke]

> 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
> 

Ok