RFR(S): 8201532: Update idom to get correct dom depth calculation

[Roland Westrelin]

Hi Nils,

> This problem started show up due to loop strip mining and ZGC barriers. 
> ZGC loadbarriers can be moved out of loops, that increases the loop 
> depth of the loop node.

When moving the loadbarrier out of loop, it shouldn't be needed to
increase the loop depth. If it is moved between the outer strip mined
loop and the counted loop, the dom depth of the barrier and its control
projection should be set to the dom depth of the outer strip mined
loop. The code that deals with domination (is_dominator() etc.) can handle
several control nodes with the same dom depth.

Roland.