RFR: 8217856: ZGC: Break out C2 matching rules into separate AD file

[Per Liden]

Break out ZGC specific C2 matching rules into separate AD file. This 
patch also introduces the z_load_barrier_slow_reg() function to avoid 
some code duplication.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217856
Webrev: http://cr.openjdk.java.net/~pliden/8217856/webrev.0

/Per