fix corner case in evaluate (#5559)

fixes #5558

lib/compress.js

@@ -5030,6 +5030,7 @@ Compressor.prototype.compress = function(node) {
                     if (def.undeclared) return this;
                     if (def.last_ref !== lhs) return this;
                     if (def.single_use == "m") return this;
+                    if (this.right.has_side_effects(compressor)) return this;
                 }
             }
             var op = this.operator;