nir: lower-io-types pass

nir: lower-io-types pass

A pass to lower complex (struct/array/mat) inputs/outputs to primitive
types.  This allows, for example, linking that removes unused components
of a larger type which is not indirectly accessed.

In the near term, it is needed for gallium (mesa/st) support for NIR,
since only used components of a type are assigned VBO slots, and we
otherwise have no way to represent that to the driver backend.  But it
should be useful for doing shader linking in NIR.

v2: use glsl_count_attribute_slots() rather than passing a type_size
    fxn pointer

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>