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/Makefile.sources

@@ -295,6 +295,7 @@ NIR_FILES = \
 	nir/nir_search_helpers.h \
 	nir/nir_serialize.c \
 	nir/nir_serialize.h \
+	nir/nir_split_per_member_structs.c \
 	nir/nir_split_var_copies.c \
 	nir/nir_sweep.c \
 	nir/nir_to_lcssa.c \