glsl: s/unsigned/glsl_base_type/ in glsl type code (v2)

glsl: s/unsigned/glsl_base_type/ in glsl type code (v2)

Declare glsl_type::sampled_type as glsl_base_type as we do for the
base_type field.  And make base_type a bitfield to save a few bytes.

Update glsl_type constructor to take glsl_base_type instead of unsigned
and pass GLSL_TYPE_VOID instead of zero.

No Piglit regressions with llvmpipe.

v2:
- Declare both base_type and sampled_type as 8-bit fields
- Use the new ASSERT_BITFIELD_SIZE() macro.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>