The needs of this pass are ever so slightly more than what
nir_opt_algebraic can do. :( Specifically, it needs to be able to look
at the relationship of constant values used in an expression tree.
v2: Add nir_mov_alu to handle swizzles on the original sources.
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19968>