pan/bi: Fix bi_find_loop_blocks for single block loops

Fixes: 6535a3b6 ("pan: Fix bi_find_loop_blocks")
Reviewed-by: Eric R. Smith <eric.smith@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39018>

src/panfrost/compiler/bifrost/bifrost_compile.c

@@ -7064,5 +7064,8 @@ bi_find_loop_blocks(const bi_context *ctx, bi_block *header, BITSET_WORD *out)
     */
    __bitset_and(out, out, dominators, BITSET_WORDS(ctx->num_blocks));
 
+   /* The header is not a predecessor of itself, so add it separately. */
+   BITSET_SET(out, header->index);
+
    ralloc_free(dominators);
 }