RFR: Introduce ShenandoahSelfFixing flag

[Roman Kennke]

> On 7/3/19 1:40 PM, Roman Kennke wrote:
>> http://cr.openjdk.java.net/~rkennke/sfx-flag/webrev.00/
> 
> Looks good. I wonder if that check should be in ShenandoahBarrierC2Support::get_load_addr itself.

Yes, that was my initial thought too. But then it would uselessly create
the visited set, and check the flag for each recursive call too. It
doesn't hurt much, but this way seems cleaner. The block could be
extracted to another method though...?

Roman