These are registerful versions of core nir_src/nir_dest which will become
SSA-only soon enough, and modifierful versions of nir_alu_src/nir_alu_dest.
The latter will let us remove modifiers from nir_alu_instr finally.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23089>
