nir: Add nir_load_interpolated_input lowering code.

nir: Add nir_load_interpolated_input lowering code.

Now nir_lower_io can optionally produce load_interpolated_input
and load_barycentric_* intrinsics for fragment shader inputs.

flat inputs continue using regular load_input.

v2: Use a nir_shader_compiler_options flag rather than ad-hoc boolean
    passing (in response to review feedback from Chris Forbes).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>