RFR: 8215708: ZGC: Add missing LoadBarrierNode::size_of()

[Per Liden]

LoadBarrierNode should implement size_of(). Otherwise cloning of such 
nodes is broken since only part of the object will be copied. This 
caused incorrect load barriers to be used in random places. For example, 
we could generate a weak barrier instead of a strong barrier, because 
the _weak member was not properly initialized when cloned.

This patch also implements three other methods (cmp, adr_type and 
match_edge) with an immediate call to ShouldNotReachHere(). This is a 
pure safety net to catch any misuse of these. These should never be 
called, but if they are called today we might not notice and instead 
silently do the wrong thing.

Bug: https://bugs.openjdk.java.net/browse/JDK-8215708
Webrev: http://cr.openjdk.java.net/~pliden/8215708/webrev.0

Testing: tier{1,6,7}

/Per