Similar to nir_lower_alu_width(), the callback can return the desired number of components for a phi, or 0 for no lowering. The previous behavior of nir_lower_phis_to_scalar() with lower_all=true can be elicited via nir_lower_all_phis_to_scalar() while the previous behavior with lower_all=false now corresponds to nir_lower_phis_to_scalar() with NULL callback. Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io> Reviewed-by: Mel Henning <mhenning@darkrefraction.com> Reviewed-by: Georg Lehmann <dadschoorse@gmail.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35783>