[RDF] Create individual phi for each indivisible register

This isn't quite using register units, but it's getting close. The phi
generation is driven by register units, but each phi still contains a
reference to a register, potentially with a mask that amounts to a unit.
In cases of explicit register aliasing this may still create phis with
references that are aliased, whereas separate phis would ideally contain
disjoint references (this is all within a single basic block).

Previously phis used maximal registers, now they use minimal. This is a
step towards both, using register units directly, and a simpler liveness
calculation algorithm. The idea is that a phi cannot reach a reference
to anything smaller than the phi itself represents. Before there could
be a phi for R1_R0, now there will be two for this case (assuming R0 and
R1 have one unit each).