fix corner case in functions (#3403)

fixes #3402

lib/compress.js

@@ -3682,6 +3682,7 @@ merge(Compressor.prototype, {
                         } else if (compressor.option("functions")
                             && def.value === def.name.fixed_value()
                             && def.value instanceof AST_Function
+                            && !(def.value.name && def.value.name.definition().assignments)
                             && can_rename(def.value, def.name.name)
                             && (!compressor.has_directive("use strict") || parent instanceof AST_Scope)) {
                             AST_Node.warn("Declaring {name} as function [{file}:{line},{col}]", template(def.name));
@@ -6301,6 +6302,13 @@ merge(Compressor.prototype, {
                 expression: self.left
             });
         }
+        if (!compressor.option("ie8") && self.left instanceof AST_Symbol && self.left.is_immutable()) {
+            return (self.operator == "=" ? self.right : make_node(AST_Binary, self, {
+                operator: self.operator.slice(0, -1),
+                left: self.left,
+                right: self.right
+            })).optimize(compressor);
+        }
         return self;
 
         function in_try(level, node) {