Eng 6396 upsert ad hoc

Just to be clear, you could have something like
```SQL
UPSERT INTO tab SELECT * FROM t1, t2, t3...
```
But that code that handles `INSERT INTO ... SELECT` should work fine for `UPSERT` too.  I think it's safe to remove the 1-table check you have here.  Seems like you really only need to PK check here.