RFR: JDK-8231583: Shenandoah: Fix register clash in SBSA::resolve_forwarding_pointer() borrowing

[Roman Kennke]

Yeah, looks good. I also tested this with the original failing testcase,
and it was good.

Thanks,
Roman

> On 9/30/19 4:57 PM, Roman Kennke wrote:
>> http://cr.openjdk.java.net/~rkennke/JDK-8231583/webrev.01/
> 
> I believe at this point we should be doing a very safe change that does not include collateral cleanups:
>   http://cr.openjdk.java.net/~shade/8231583/webrev.01/
> 
> Testing: hotspot_gc_shenandoah {x86_32, x86_64, aarch64}
>