fix cascade of evaluate optimisation (#1654)

Operator has changed, so break out from rest of the rules.

fixes #1649

lib/compress.js

@@ -3292,6 +3292,7 @@ merge(Compressor.prototype, {
                         left: self.left,
                         right: self.right.expression
                     });
+                    break;
                 }
                 // -a + b => b - a
                 if (self.left instanceof AST_UnaryPrefix
@@ -3303,6 +3304,7 @@ merge(Compressor.prototype, {
                         left: self.right,
                         right: self.left.expression
                     });
+                    break;
                 }
               case "*":
                 associative = compressor.option("unsafe_math");