RFR (XS): 8212753: Improve oopDesc::forward_to_atomic

[Kim Barrett]

> On Oct 22, 2018, at 9:02 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8212753
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8212753/webrev/
> Testing:
> hs-tier1-3
> 
> Thanks,
>  Thomas

There’s an additional change to the G1 code here:
src/hotspot/share/gc/g1/g1ParScanThreadState.cpp
 313     } else if (!obj->is_typeArray()) {

which seems unrelated to the described change and unmentioned in the RFR.
Is this change intentional here?  Is this a measurable optimization? 

For the CMS-conditionalized asserts in forward_to_atomic, please add a
comment that CMS may CAS in a special pseudo-oop value.