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

[Per Liden]

I ran this through tier{1,2,3,4,5} over night without running into the 
assert we occasionally saw before. So the change seems good.

/Per

On 06/15/2018 11:38 AM, Nils Eliasson wrote:
> 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
>