Implement: fill_phi_node_list()
Scan each of the list of paths in the src, and reduce the list to
a list of immediately/first previous nodes with assocated node that assigned the register.
Also do sanity checks on the path nodes lists based on first_prev_node.
This reduces the PHI to a format similar to that used in LLVM.
Check phi of the prev_node when searching for the phi src.