nir: Add a concept of per-member structs and a lowering pass

This adds a concept of "members" to a variable with an interface type.
It allows you to specify the full variable data for each member of the
interface instead of once for the variable.  We also add a lowering pass
to lower those variables to a sequence of variables and rewrite all the
derefs accordingly.

Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/meson.build

@@ -180,6 +180,7 @@ files_libnir = files(
   'nir_search_helpers.h',
   'nir_serialize.c',
   'nir_serialize.h',
+  'nir_split_per_member_structs.c',
   'nir_split_var_copies.c',
   'nir_sweep.c',
   'nir_to_lcssa.c',