glsl: Add an optimization pass to simplify discards.

NOTE: This is a candidate for the 7.9 branch.

src/glsl/glsl_parser_extras.cpp

@@ -716,6 +716,7 @@ do_common_optimization(exec_list *ir, bool linked, unsigned max_unroll_iteration
    }
    progress = do_structure_splitting(ir) || progress;
    progress = do_if_simplification(ir) || progress;
+   progress = do_discard_simplification(ir) || progress;
    progress = do_copy_propagation(ir) || progress;
    if (linked)
       progress = do_dead_code(ir) || progress;