RFR: 8079315: UseCondCardMark broken in conjunction with CMS precleaning

[Andrew Haley]

I've added StoreLoad barriers everywhere they're needed.

http://cr.openjdk.java.net/~aph/8079315/

This patch depends on the patch for 8078438, which is still not
committed.  http://cr.openjdk.java.net/~shade/8078438/webrev.02/

This is x86 only, will do AArch64 parts in a separate patch once
we've agreed on this.

Andrew.