RFR: ZGC: C2: Don't split loadbarriers past self referencing phi nodes

[Nils Eliasson]

Hi,

I found a case when we are trying to split a loadbarrier past a phi 
nodes which has one input that is itself. This is a workaround to just 
skip those. Later we might want to try to remove these nodes earlier in 
loop optimizations, but that is an upstream issue.

http://cr.openjdk.java.net/~neliasso/zgc/dont_split/webrev.01/

Regards,

Nils