[tree-ssa] Insert on edge comment

[law@redhat.com]

In message <3ED7E4C7.69416EBC@redhat.com>, "Vladimir N. Makarov" writes:
 >I am ignorant too (never looked at ssa-tree branch).  As I understand the
 >problem is in phi nodes which are based on dominator frontier analysis.
 >Insertion of new BB is simple for general IR but for SSA it can significantly
 >change dominator frontiers for many blocks and as consequence phi-nodes in
 >the dominator frontiers. So updating phi-nodes is not a trivial task.
That's certainly true as well.  If you look at how we do this at the
RTL level, you'll find that we don't split edges while in SSA form.  
Instead we queue things (such as edge insertions) and split the edge
after we no longer need dominance info and PHI nodes.

jeff