i965: Use nir_lower_passthrough_edgeflags

Now that there's a common NIR pass, there's no point in us doing this in the back-end anymore. In order to use this pass in i965, we do have to make one tiny change. Gallium runs the pass after assigning input and output locations and so needs the pass to respect those locations and num_inputs. i965, however, runs it before any location assignment or I/O lowering so we don't care. We do, however, need the pass to succeed with num_inputs == 0 because we set that later.