RFR (S) 7901096: Blackhole.consume(primitive) do two volatile reads

[Aleksey Shipilev]

Hi,

I would like a second pair of eyes on this patch:
 https://bugs.openjdk.java.net/browse/CODETOOLS-7901096
 http://cr.openjdk.java.net/~shade/7901096/webrev.00/

The patch makes the memory effects consistent with
Blackhole.consume(Object). It improves the performance on ARM
(expected), and keeps it the same on x86 (expected):
 http://cr.openjdk.java.net/~shade/7901096/performance.txt

Thanks,
-Aleksey.