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.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10809>

src/intel/compiler/brw_nir.c

@@ -600,7 +600,7 @@ brw_nir_optimize(nir_shader *nir, const struct brw_compiler *compiler,
       OPT(nir_copy_prop);
 
       if (is_scalar) {
-         OPT(nir_lower_phis_to_scalar);
+         OPT(nir_lower_phis_to_scalar, false);
       }
 
       OPT(nir_copy_prop);