c15498afbe
Some uses don't have any post-dominator. An example is an atomic that
feeds itself in a loop. No instruction immediately post-dominates
the result of such an atomic because no instruction can strictly
post-dominate itself. This handles that case generally by setting
the root node as the post-dominator for instructions that can't be
reordered.
Fixes: ba54099dce - nir: add a utility computing post-dominance of SSA uses
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28436>