nir: Assert the nir_src union is used safely

nir: Assert the nir_src union is used safely

It is undefined behaviour in C to read a different member of a union than was
written. Nothing in-tree should be using this behaviour with the nir_src union:
nir_if should never be read as nir_instr and vice versa. Assert this.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Acked-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24671>