agx: Allow loop headers without later preds

These happen for loops that break after exactly 1 iteration, unconditionally.
Fixes validation splat in dEQP-VK.glsl.switch.conditional_fall_through_2_uniform_vertex

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24847>

src/asahi/compiler/agx_validate.c

@@ -189,7 +189,7 @@ agx_validate_predecessors(agx_block *block)
          has_later_preds = true;
    }
 
-   if (block->loop_header != has_later_preds)
+   if (has_later_preds && !block->loop_header)
       return false;
 
    /* Successors and predecessors are found together */