RFR 8233339: Shenandoah: Centralize load barrier decisions into ShenandoahBarrierSet

[Zhengyu Gu]

Right now, the decisions on, if a load barrier needs load reference 
barrier, if so, what kind? and if the reference needs to be kept alive, 
are scattered inside interpreter/c1/2 load barrier code, which is hard 
to make them consistent.

I would like to centralize the decision making into 
ShenandoahBarrierSet, so them can be consistent and easy to maintain.

Bug: https://bugs.openjdk.java.net/browse/JDK-8233339
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8233339/webrev.00/index.html

Test:
   hotspot_gc_shenandoah (fastdebug and release)
   x86_64 and x86_32 on Linux
   AArch64 on Linux

Thanks,

-Zhengyu