[12] RFR(XS): 8215755: ZGC: split_barrier_thru_phi: check number of inputs of phi

[Nils Eliasson]

Hi,

This fix adds a check of number of inputs before a check of a specific 
input.

Bug: https://bugs.openjdk.java.net/browse/JDK-8215755

Webrev: http://cr.openjdk.java.net/~neliasso/8215755

Please review,

Nils