Review request: 8026707: JDK-8026391 broke the optimized build target

[Stefan Karlsson]

http://cr.openjdk.java.net/~stefank/8026707/webrev.00/

Metachunk::mangle() was incorrectly moved being guarded by NOT_PRODUCT() 
to being guarded by ASSERT. The patch reverts that change.

thanks,
StefanK