prefetch and ring_buffer don't interact well

[abadams]

Description

If you prefetch and ring_buffer the same Func, it only prefetches ring slot zero. The repro below produces the IR:

   allocate producer[int32 * 17 * 1 * 2]
   for (consumer.s0.x.xo, 0, (consumer.extent.0/16) + -1) {
    produce producer {
     let t330 = (consumer.s0.x.xo*16) + ((consumer.min.0 + consumer.min.1) + consumer.s0.y.rebased)
     for (producer.s0.x.rebased, 0, 16) {
      producer[((consumer.s0.x.xo % 2)*17) + producer.s0.x.rebased] = producer.s0.x.rebased + t330
     }
    }
    consume producer {
     let t333 = (consumer.s0.x.xo*16) + (consumer.s0.y.rebased*consumer.stride.1)
     for (consumer.s0.x.xi, 0, 15) {
      if (consumer.s0.x.xi <= 11) {
       prefetch(producer, consumer.s0.x.xi + 4, 1, 16)
      }
      consumer[consumer.s0.x.xi + t333] = 
        let t355 = ((consumer.s0.x.xo % 2)*17) + consumer.s0.x.xi in
        producer[t355] + producer[t355 + 1]

     }
    }
   }

Nothing about the prefetch node names xo, even though the actual consumption toggles based on it.

Reproducing case

#include "Halide.h"
using namespace Halide;

int main() {
    Func producer("producer"), consumer("consumer");
    Var x("x"), y("y"), xo("xo"), xi("xi");

    producer(x, y) = x + y;
    consumer(x, y) = producer(x, y) + producer(x + 1, y);

    consumer.compute_root().split(x, xo, xi, 16, TailStrategy::RoundUp);
    producer.compute_at(consumer, xo)
        .hoist_storage(consumer, y)
        .ring_buffer(2);
    consumer.prefetch(producer, xi, xi, 4);

    consumer.compile_to_lowered_stmt("repro.stmt", {}, StmtOutputFormat::Text,
                                     Target("host-no_asserts-no_bounds_query-no_runtime"));
    Buffer<int> out = consumer.realize({64, 4});
    return 0;
}