Link Phi instructions in a module.
For Phi instructions, the Salt->Llvm converter just fills in the source
block label of each variable to be merged with
undef. We need to add
the real block label of the in-edge that defines each variable.
We build a graph of each block, work out the in-edges due to branches, and fill in the real block labels by back tracing the in-edges until we find the node that defines each variable.