nir: Add a nir->info.uses_interp_var_at_offset flag.

I've added this to nir_gather_info(), but also to glsl_to_nir() as a
temporary measure, since the i965 GL driver today doesn't use
nir_gather_info() yet.

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

src/compiler/nir/nir.h

@@ -1716,6 +1716,9 @@ typedef struct nir_shader_info {
    /* Whether or not this shader ever uses textureGather() */
    bool uses_texture_gather;
 
+   /** Whether or not this shader uses nir_intrinsic_interp_var_at_offset */
+   bool uses_interp_var_at_offset;
+
    /* Whether or not this shader uses the gl_ClipDistance output */
    bool uses_clip_distance_out;