RFR (S) 8222766: Shenandoah: streamline post-LRB CAS barrier

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8222766

Webrev:
  http://cr.openjdk.java.net/~shade/8222766/webrev.02/

Now that LRB switched Shenandoah to strong to-space invariant, we don't need the retry loop in CAS
barrier. Instead, we can "just" fixup the memory pointer before retrying. Added more comments to
explain how that works. Also, AArch64 snippet was synced up with x86_64 version, and switched to
"macro" cmpxchg, which gives us cmpxchg instructions when LSE is enabled.

Testing: Linux {x86_64, aarch64}: hotspot_gc_shenandoah, full jcstress

-- 
Thanks,
-Aleksey