RFR: Fix weakref precleaning

[Aleksey Shipilev]

On 05/31/2017 09:02 PM, Roman Kennke wrote:
> Oops. Shouldn't do stuff in a rush.
> http://cr.openjdk.java.net/~rkennke/fixpreclean/webrev.01/

Remaining nits:

 *) Since you removed the branch that made rp possibly NULL, you can use
ReferenceProcessorIsAliveMutator again:

 600     ReferenceProcessor* rp = sh->ref_processor();;
 601
 602     ReferenceProcessorMaybeNullIsAliveMutator fix_alive(rp,
ShenandoahHeap::heap()->is_alive_closure());

 *) Also notice ";;" above

Other than that looks good.

-Aleksey