RFR: Introduce ShenandoahSelfFixing flag

[Roman Kennke]

I propose to add a flag ShenandoahSelfFixing that turns on/off
self-fixing references via LRB. Defaulting to on because it doesn't seem
to harm most programs that I have looked at, and seems to improve
latency by helping avoid the LRB slow-paths. It'll be useful for
differential performance testing and recovering throughput in the rare
cases where it matters. It works by passing NULL instead of the
load-addr in C2 generation. This fully restores performance
characteristics of baseline (e.g. jdk/jdk)

http://cr.openjdk.java.net/~rkennke/sfx-flag/webrev.00/

Testing: hotspot_gc_shenandoah

Ok for sh/jdk ?

Roman