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/compiler/nir/meson.build

@@ -120,11 +120,11 @@ files_libnir = files(
   'nir_lower_passthrough_edgeflags.c',
   'nir_lower_patch_vertices.c',
   'nir_lower_phis_to_scalar.c',
-  'nir_lower_read_invocation_to_scalar.c',
   'nir_lower_regs_to_ssa.c',
   'nir_lower_returns.c',
   'nir_lower_samplers.c',
   'nir_lower_samplers_as_deref.c',
+  'nir_lower_subgroups.c',
   'nir_lower_system_values.c',
   'nir_lower_tex.c',
   'nir_lower_to_source_mods.c',