glsl2: Early return with visit_continue in loop_analysis::visit(ir_dereference_variable *)

Returning early with visit_continue_with_parent prevented the
then-statements and else-statements of if-statements such as the
following from being processed:

	  if (some_var) { ... } else { ... }

Fixes piglit test case glsl-fs-loop-nested-if and bugzilla #30030.

src/glsl/loop_analysis.cpp

@@ -154,7 +154,7 @@ loop_analysis::visit(ir_dereference_variable *ir)
    /* If we're not somewhere inside a loop, there's nothing to do.
     */
    if (this->state.is_empty())
-      return visit_continue_with_parent;
+      return visit_continue;
 
    loop_variable_state *const ls =
       (loop_variable_state *) this->state.get_head();