RFR (XS): 8211046: Forced data dependencies serve no purpose on x86

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8211046
webrev: http://cr.openjdk.java.net/~dholmes/8211046/webrev/

The fix for this is to look at the code prior to 8188764 and delete all 
the os::is_MP code, and keep only the !is_MP code.

Testing: tier 1-3

Thanks,
David