nir: Add a pass to lower TES patch_vertices intrinsics to a constant.

In Vulkan, we always have both the TCS and TES available in the same
pipeline, so we can simply use the TCS OutputVertices execution mode
value as the TES PatchVertices built-in.

For GLSL, we handle this in the linker.  But we could use this pass
in the case when both TCS and TES are linked together, if we wanted.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -2490,6 +2490,7 @@ void nir_lower_two_sided_color(nir_shader *shader);
 void nir_lower_clamp_color_outputs(nir_shader *shader);
 
 void nir_lower_passthrough_edgeflags(nir_shader *shader);
+void nir_lower_tes_patch_vertices(nir_shader *tes, unsigned patch_vertices);
 
 typedef struct nir_lower_wpos_ytransform_options {
    int state_tokens[5];