nir: add always_active_io to nir variable

Will be used in nir link pass to decided if we can remove a varying
or not.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>

src/compiler/nir/nir.h

@@ -191,6 +191,16 @@ typedef struct nir_variable {
       unsigned patch:1;
       unsigned invariant:1;
 
+      /**
+       * When separate shader programs are enabled, only input/outputs between
+       * the stages of a multi-stage separate program can be safely removed
+       * from the shader interface. Other input/outputs must remains active.
+       *
+       * This is also used to make sure xfb varyings that are unused by the
+       * fragment shader are not removed.
+       */
+      unsigned always_active_io:1;
+
       /**
        * Interpolation mode for shader inputs / outputs
        *