nir: Add intrinsics for tessellation shader system values.

nir: Add intrinsics for tessellation shader system values.

nir_intrinsic_load_patch_vertices_in corresponds to gl_PatchVerticesIn,
a special input in both the TCS and TES stages.

nir_intrinsic_load_tess_coord corresponds to gl_TessCoord, a special
tessellation evaluation shader input.

nir_intrinsic_load_tess_level_outer/inner correspond to the
gl_TessLevelOuter[] and gl_TessLevelInner[] evaluation shader inputs,
which we treat as system values because they're stored specially.
(These intrinsics are only for the TES - the TCS uses output variables.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>