stay safe with constants in IE8- (#1547)

- `undefined` etc. can be redefined at top-level for IE8-, so disable related optimisations
- fixed `--support-ie8` catch mangle bug

lib/compress.js

@@ -3348,7 +3348,9 @@ merge(Compressor.prototype, {
             return def;
         }
         // testing against !self.scope.uses_with first is an optimization
-        if (self.undeclared() && !isLHS(self, compressor.parent())
+        if (compressor.option("screw_ie8")
+            && self.undeclared()
+            && !isLHS(self, compressor.parent())
             && (!self.scope.uses_with || !compressor.find_parent(AST_With))) {
             switch (self.name) {
               case "undefined":