RFR: 8217503: ZGC: Fix fall through bug in ZBarrierSetC2::escape_add_final_edges()

[Per Liden]

The switch statement in ZBarrierSetC2::escape_add_final_edges() has a 
fall through bug, letting the 
Op_LoadBarrierSlowReg/Op_LoadBarrierWeakSlowReg case fall through into 
the Op_Proj case.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217503
Webrev: http://cr.openjdk.java.net/~pliden/8217503/webrev.0

/Per