Sadly need to poke a bit in the src internals to avoid using yet another
heap allocated datastructure.
Fixes: 5251548572 ("nir: Add a nir_instr_remove that recursively removes dead code.")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/5323
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12726>