Precompute fails if there are non-iname dependencies in the rule invocation.

knl = lp.make_kernel(
        "{[i,j,k]: 0<=i<100 and 0<=j<32 and 0<=k<8}",
        """
        for i
          for j
            x[j] = 3.14 * (i + j)
            y[j] = 0
            for k
              y[j] = y[j] + L[map[i], k, j] *x[j]
            end
            out[i, j] = y[j]
          end
        end
        """,
        [
            lp.GlobalArg('L', dtype=np.float64, shape=(1, 8, 32)),
            lp.GlobalArg('map', dtype=np.int32, shape=lp.auto),
            lp.TemporaryVariable('x', dtype=np.float64, shape=(32,)),
            lp.TemporaryVariable('y', dtype=np.float64, shape=(32,)),
            '...'],
        seq_dependencies=True,
        lang_version=(2018, 2)
        )

knl = lp.add_prefetch(knl, "L", sweep_inames="j,k",
    fetch_outer_inames="i", temporary_scope=lp.AddressSpace.LOCAL)

fails as --

File "loopy/loopy/transform/precompute.py", line 531, in precompute                                                                                    
  raise RuntimeError("no invocations of '%s' found" % subst_name)
RuntimeError: no invocations of 'L_fetch_rule' found

Reason: this FIXME.