agx: Assert vertex_id, instance_id are VS-only

We can get them in other shaders transiently due to merging shader stages, but
they need to be lowered since the hardware versions only exist in the hardware
VS. So the compiler should never see them outside VS.

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

src/asahi/compiler/agx_compile.c

@@ -1043,9 +1043,11 @@ agx_emit_intrinsic(agx_builder *b, nir_intrinsic_instr *instr)
                             AGX_ICOND_UEQ);
 
    case nir_intrinsic_load_vertex_id:
+      assert(b->shader->stage == MESA_SHADER_VERTEX);
       return agx_mov_to(b, dst, agx_abs(agx_vertex_id(b)));
 
    case nir_intrinsic_load_instance_id:
+      assert(b->shader->stage == MESA_SHADER_VERTEX);
       return agx_mov_to(b, dst, agx_abs(agx_instance_id(b)));
 
    case nir_intrinsic_load_preamble: