nir: Add a new subgroups lowering pass

This commit pulls nir_lower_read_invocations_to_scalar along with most
of the guts of nir_opt_intrinsics (which mostly does subgroup lowering)
into a new nir_lower_subgroups pass.  There are various other bits of
subgroup lowering that we're going to want to do so it makes a bit more
sense to keep it all together in one pass.  We also move it in i965 to
happen after nir_lower_system_values to ensure that because we want to
handle the subgroup mask system value intrinsics here.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/intel/compiler/brw_compiler.c

@@ -57,7 +57,6 @@ static const struct nir_shader_compiler_options scalar_nir_options = {
    .lower_unpack_snorm_4x8 = true,
    .lower_unpack_unorm_2x16 = true,
    .lower_unpack_unorm_4x8 = true,
-   .lower_subgroup_masks = true,
    .max_subgroup_size = 32,
    .max_unroll_iterations = 32,
 };
@@ -80,7 +79,6 @@ static const struct nir_shader_compiler_options vector_nir_options = {
    .lower_unpack_unorm_2x16 = true,
    .lower_extract_byte = true,
    .lower_extract_word = true,
-   .lower_vote_trivial = true,
    .max_unroll_iterations = 32,
 };
 
@@ -99,7 +97,6 @@ static const struct nir_shader_compiler_options vector_nir_options_gen6 = {
    .lower_unpack_unorm_2x16 = true,
    .lower_extract_byte = true,
    .lower_extract_word = true,
-   .lower_vote_trivial = true,
    .max_unroll_iterations = 32,
 };