agx: Set flag on last st_vary instruction

Not sure what the point is but let's match the blob.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10582>

src/asahi/compiler/agx_compile.c

@@ -556,6 +556,17 @@ emit_cf_list(agx_context *ctx, struct exec_list *list)
    return start_block;
 }
 
+static void
+agx_set_st_vary_final(agx_context *ctx)
+{
+   agx_foreach_instr_global_rev(ctx, I) {
+      if (I->op == AGX_OPCODE_ST_VARY) {
+         I->last = true;
+         return;
+      }
+   }
+}
+
 static void
 agx_print_stats(agx_context *ctx, unsigned size, FILE *fp)
 {
@@ -797,6 +808,9 @@ agx_compile_shader_nir(nir_shader *nir,
 
    agx_ra(ctx);
 
+   if (ctx->stage == MESA_SHADER_VERTEX)
+      agx_set_st_vary_final(ctx);
+
    if (agx_debug & AGX_DBG_SHADERS && !skip_internal)
       agx_print_shader(ctx, stdout);