nir/vars_to_ssa: Use the new nir_phi_builder helper

nir/vars_to_ssa: Use the new nir_phi_builder helper

The efficiency should be approximately the same.  We do a little more work
per phi node because we have to sort the predecessors.  However, we no
longer have to walk the blocks a second time to pop things off the stack.
The bigger advantage, however, is that we can now re-use the phi placement
and per-block SSA value tracking in other passes.