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

[Nils Eliasson]

Hi,

In testing we hit an assert in insert_post_loop that the dom depth was 
wrong. I tracked down the cause to be a node replacement in counted 
loops that only does a lazy_replace. Lazy replace is enough when all 
later transformation are using the idom()-getter (it checks the 
new-to-old-mapping in _nodes). In insert_post_loop we use dom depth for 
an assert. The dom-depth have been recalculated, but 
recompue_dom_depth-call doesn't use the lazy mappings and computes dom 
depth on the old and stale data.

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. Loop strip mining adds an outer loop also affect 
the loop depth. If there exists previous lazy updates to the idom when 
those optimizations are done, the dom depth recalculation will be wrong, 
and asserts can be hit.

Bug: https://bugs.openjdk.java.net/browse/JDK-8201532

Webrev: http://cr.openjdk.java.net/~neliasso/8201532/webrev.01/

Regards,

Nils