Here's a much more complete description of how I do SSA, beyond just how I do Phis.
This describes how I do SSA form, which avoids the need to have any coupling between CFG data structures and SSA data structures.
Let's first define a syntax for SSA and some terminology. Here's an example SSA node:
A = Add(B, C)
In reality, this will be a single object in your in-memory representation, and the names are really addresses of those objects. So, this node has an "implicit variable" called A; it's the variable that is implicitly assigned to when you execute the node. If you then do: