RFR (XS): 8191915: JCK tests produce incorrect results with C2

[Rickard Bäckman]

Hi, can I please have this change reviewed?

Summary is that we had an overflow check for mulitplyExact(long, long)
intrinsic that relied on undefined behaviour. clang / llvm started
optimizing that so that if a = b * c then a / b must be equal to c and
removed half of the overflow check. 

Changing the multiplication and division to be unsigned.

Webrev: http://cr.openjdk.java.net/~rbackman/8191915/
Bug: https://bugs.openjdk.java.net/browse/JDK-8191915

Thanks
/R