nir/lower_phis_to_scalar: Add "lower_all" option

We don't want to have to deal with vector phis in freedreno, because vectors are always split/unsplit around vectorized instructions anyways, and the stated reason for not scalarising them (it hurting coalescing) won't apply to us because we won't be using nir_from_ssa. Add this option so that we don't have to do the equivalent thing while translating from NIR.

I'm splitting this out from !9842 (merged) because it's been reviewed already and it's a frequent source of random rebase conflicts, but also as a heads-up to everyone not interested in that MR in case anyone is opposed.