RFR (M): (x86) Fix up and refactor store checks

[Aleksey Shipilev]

On 04/25/2017 08:31 PM, Aleksey Shipilev wrote:
> Hi,
> 
> With Verify{Reads|Writes} purged, the only way for us to verify store invariants
> is compiler-assisted checks. Which means we would like make them work better
> than they are now.
> 
> This is the update:
>   http://cr.openjdk.java.net/~shade/shenandoah/store-check-refactor/webrev.01/

Roman had remembered (offline) that there is a reason for a combined check: we
should only do val_checks for targets in heap, because we accept from-refs
stores to roots, as they would be fixed up later.

Accepted this by doing a light-weight in_heap_check in val_check:
  http://cr.openjdk.java.net/~shade/shenandoah/store-check-refactor/webrev.02/

Thanks,
-Aleksey