RFR (S) 8222766: Shenandoah: streamline post-LRB CAS barrier (x86)

[Roman Kennke]

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8222766
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8222766/webrev.07/
> 
> I hope the comments in the new code are self-explanatory. This rewrite allows us to ditch
> resolve_fwd_ptr and its awkward borrowing scheme. Since it is removing two of three fwdptr resolves,
> it also considerably improves the generated code quality for CAS -- which is measurable on
> microbenchmarks.
> 
> The AArch64 counterpart comes later in JDK-8232782.
> 
> Compare:
>   https://cr.openjdk.java.net/~shade/8222766/shenandoah-cas-before.perfasm
>   https://cr.openjdk.java.net/~shade/8222766/shenandoah-cas-after.perfasm
> 
> Testing: {x86_32, x86_64} hotspot_gc_shenandoah; jcstress runs

Looks good to me!

Thanks,
Roman