enable collapse_vars & reduce_vars by default

- fix corner cases in `const` optimisation
- deprecate `/*@const*/`

fixes #1497
closes #1498

test/compress/evaluate.js

@@ -602,6 +602,7 @@ unsafe_prototype_function: {
 call_args: {
     options = {
         evaluate: true,
+        reduce_vars: true,
     }
     input: {
         const a = 1;