nir/builder_opcodes: Do not generate empty intrinsic indices

Gets rid of all the

struct nir_*_indices {
   int _; /* exists to avoid empty initializers */
};

declarations. 14293 loc -> 12900 loc

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Friedrich Vock <friedrich.vock@gmx.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23906>

src/compiler/nir/nir_builder_opcodes_h.py

@@ -62,12 +62,14 @@ nir_${name}(nir_builder *build, ${src_decl_list(opcode.num_inputs)})
 % endfor
 
 % for name, opcode in sorted(INTR_OPCODES.items()):
+% if opcode.indices:
 struct _nir_${name}_indices {
    int _; /* exists to avoid empty initializers */
 % for index in opcode.indices:
    ${index.c_data_type} ${index.name};
 % endfor
 };
+% endif
 % endfor
 
 <%