RFR: Assembler write barriers should consistently check for forwarded objects

[Roman Kennke]

Am 10.07.2018 um 10:56 schrieb Aleksey Shipilev:
> http://cr.openjdk.java.net/~shade/shenandoah/sbs-forwarded-check/webrev.01/
> 
> This fixes the bug in write_barrier_impl in x86_64: we should check either for 0, or for
> HAS_FORWARDED when deciding to omit the read barrier. C1 write barrier tests HAS_FORWARDED and it
> seems most efficient when coalescing is not available. So, the rest homogenizes the check shape
> across architectures.
> 
> Testing: tier3_gc_shenandoah (x86_64), build (aarch64)
> 
> Thanks,
> -Aleksey
> 

Can you hold it back until I pushed the consolidation?

Roman