pan/bi: Add helpers for working with singletons

Clauses with exactly one instruction (not canonical terminology to my
knowledge, but the notation is suggestive). Since these are isomorphic
to the instructions themselves, we want helpers to go between the forms.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7206>
1 file changed