fix corner case in conditionals (#4948)

fixes #4947

lib/compress.js

@@ -11412,6 +11412,7 @@ merge(Compressor.prototype, {
 
         function is_tail_equivalent(consequent, alternative) {
             if (consequent.TYPE != alternative.TYPE) return;
+            if (consequent.optional != alternative.optional) return;
             if (consequent instanceof AST_Call) {
                 if (arg_diff(consequent, alternative) != -1) return;
                 return consequent.TYPE != "Call"