agx: Assert that barriers are not used in the preamble

It is nonsensical and confuses the hardware.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23998>

src/asahi/compiler/agx_compiler.h

@@ -378,6 +378,8 @@ typedef struct agx_block {
 typedef struct {
    nir_shader *nir;
    gl_shader_stage stage;
+   bool is_preamble;
+
    struct list_head blocks; /* list of agx_block */
    struct agx_shader_info *out;
    struct agx_shader_key *key;