glsl: Add glsl LowerCsDerivedVariables option

v2:
 * Move lower flag to context constants. (Ken)

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/glsl/ir.h

@@ -2562,7 +2562,8 @@ _mesa_glsl_initialize_variables(exec_list *instructions,
 				struct _mesa_glsl_parse_state *state);
 
 extern void
-_mesa_glsl_initialize_derived_variables(gl_shader *shader);
+_mesa_glsl_initialize_derived_variables(struct gl_context *ctx,
+                                        gl_shader *shader);
 
 extern void
 _mesa_glsl_initialize_functions(_mesa_glsl_parse_state *state);