RFR: 8225716: G1 GC: Undefined behaviour in G1BlockOffsetTablePart::block_at_or_preceding

[Andrew Haley]

http://cr.openjdk.java.net/~aph/8225716-jdk8u/

This patch had to be backported with changes because current Atomic
support doesn't work for byte accesses, so volatile will suffice.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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