agx: implement load_subgroup_invocation

There's an sr.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27616>

src/asahi/compiler/agx_compile.c

@@ -1257,6 +1257,9 @@ agx_emit_intrinsic(agx_builder *b, nir_intrinsic_instr *instr)
       return NULL;
    }
 
+   case nir_intrinsic_load_subgroup_invocation:
+      return agx_get_sr_to(b, dst, AGX_SR_THREAD_INDEX_IN_SUBGROUP);
+
    case nir_intrinsic_reduce: {
       assert(nir_intrinsic_reduction_op(instr) == nir_op_iadd &&
              "other reductions todo");