I'm extremely uncertain of my history here, but my recollection is that SSA wasn't seen as practical until the development of the dominance frontier algorithm for inserting phi nodes, which seems to be 1991.
Yes - ironically, this was actually the serendipitous part - wegman and zadeck were walking and saw cytron working on a whiteboard at the end of the hall (at watson research, all the halls had open whiteboard areas at the end of each hall), and realized he had solved/or was working on the same problem they were having.
