nir/lower_io: Use load_per_vertex_input intrinsics for TCS and TES.

nir/lower_io: Use load_per_vertex_input intrinsics for TCS and TES.

Tessellation control shader inputs are an array indexed by the vertex
number, like geometry shader inputs.  There aren't per-patch TCS inputs.

Tessellation evaluation shaders have both per-vertex and per-patch
inputs.  Per-vertex inputs get the new intrinsics; per-patch inputs
continue to use the ordinary load_input intrinsics, as they already
work like we want them to.

v2: Change stage_uses_per_vertex_inputs into is_per_vertex_input(),
    which takes a variable (requested by Jason Ekstrand).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>