8205097: RFR(S): zBarrierSetC2::split_barrier_thru_phi need to update idom

[Nils Eliasson]

Hi,

This patch fixes missing updates of the idom when splitting barrier thru 
phis.

Please review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8205097

Webrev: http://cr.openjdk.java.net/~neliasso/8205097/webrev.01/

Regards,

Nils Eliasson