[tree-ssa] Insert on edge comment

[Andrew MacLeod]

By the way, it occurs to me that if you are trying to use insert_on_edge
somewhere other than my out_of_ssa pass, we have some crap to deal with.

If you insert on an edge, and it causes a block to be split, nothing
gets done to the PHI nodes. We/I will probably have to sit down someday
soon and figure out what needs to be done there. At the very least the
edge feeding the phi will have to be updated.  My new switch edge
manipulation will be a real ugly one to update.. brrr.

There may be other structures which need to be fixed, etc.


Andrew