nir: Use 8-bit types for most info fields

This shrinks nir_intrinsics.c.o from 73K to 35K and nir_opcodes.c.o from
64K to 31K on a release build.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5045>

src/compiler/nir/nir_serialize.c

@@ -1443,7 +1443,7 @@ union packed_tex_data {
    uint32_t u32;
    struct {
       enum glsl_sampler_dim sampler_dim:4;
-      nir_alu_type dest_type:8;
+      unsigned dest_type:8;
       unsigned coord_components:3;
       unsigned is_array:1;
       unsigned is_shadow:1;