glsl: disable varying packing for varying used by interpolateAt*

Currently the NIR backends depend on GLSL IR copy propagation to
fix up the interpolateAt* function params after varying packing
changes the shader input to a global. It's possible copy propagation
might not always do what we need it too, and we also shouldn't
depend on optimisations to do this type of thing for us.

I'm not sure if the same is true for TGSI, but the following
commit should re-enable packing for most cases in a safer way,
so we just disable it everywhere.

No change in shader-db for i965 (BDW)

Acked-by: Elie Tournier <elie.tournier@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/glsl/ast_function.cpp

@@ -235,6 +235,8 @@ verify_parameter_modes(_mesa_glsl_parse_state *state,
                              formal->name);
             return false;
          }
+
+         val->variable_referenced()->data.must_be_shader_input = 1;
       }
 
       /* Verify that 'out' and 'inout' actual parameters are lvalues. */