introduce unsafe_regexp (#1970)

fixes #1964

lib/compress.js

@@ -84,6 +84,7 @@ function Compressor(options, false_by_default) {
         unsafe_comps  : false,
         unsafe_math   : false,
         unsafe_proto  : false,
+        unsafe_regexp : false,
         unused        : !false_by_default,
         warnings      : true,
     }, true);
@@ -3680,7 +3681,7 @@ merge(Compressor.prototype, {
             if (fixed) {
                 if (d.should_replace === undefined) {
                     var init = fixed.evaluate(compressor);
-                    if (init !== fixed) {
+                    if (init !== fixed && (compressor.option("unsafe_regexp") || !(init instanceof RegExp))) {
                         init = make_node_from_constant(init, fixed);
                         var value = init.optimize(compressor).print_to_string().length;
                         var fn;