agx: strengthen assert

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31532>

src/asahi/compiler/agx_register_allocate.c

@@ -692,6 +692,7 @@ agx_emit_move_before_phi(agx_context *ctx, agx_block *block,
 
    agx_instr *phi = agx_phi_to(&b, agx_register_like(copy->dest, copy->src),
                                agx_num_predecessors(block));
+   assert(!copy->src.kill);
 
    agx_foreach_src(phi, s) {
       phi->src[s] = copy->src;