`graph-ssa-find-joins:`procedureGiven a list of SSA nodes, returns the list of nodes

`N`for which there are (at least) two paths`... N_0 M_0 ... M_i N`and`... N_1 P_0 ... P_j N`such that`N_0`and`N_1`are distinct members of`NODES`and the M's and P's are disjoint sets.