agx: Assert invariant stated in the comment

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24635>

src/asahi/compiler/agx_register_allocate.c

@@ -689,6 +689,7 @@ reserve_live_in(struct ra_ctx *rctx)
                 * it in until the end of the loop. Stash in the information
                 * we'll need to fill in the real register later.
                 */
+               assert(rctx->block->loop_header);
                phi->src[pred_idx] = agx_get_index(i, size);
             } else {
                /* Otherwise, we can build the phi now */