nir: Use a tagged pointer for nir_src parents

nir: Use a tagged pointer for nir_src parents

This allows us to pack the is_if boolean into the bottom bit of the parent
pointer, eliminating the boolean and hence shrinking the nir_src by 8 bytes (due
to the extra 63 bits of padding incurred in the old layout).

Because all access is forced through helpers now, this is a local change.

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>