fix corner cases in braces & sequences (#4221)

fixes #4220

lib/compress.js

@@ -2513,7 +2513,7 @@ merge(Compressor.prototype, {
                 var line = block.body[i];
                 if (line instanceof AST_Var && declarations_only(line)) {
                     decls.push(line);
-                } else if (stat) {
+                } else if (stat || line instanceof AST_Const) {
                     return false;
                 } else {
                     stat = line;

lib/output.js

@@ -990,7 +990,7 @@ function OutputStream(options) {
     /* -----[ if ]----- */
     function make_then(self, output) {
         var b = self.body;
-        if (output.option("braces")
+        if (output.option("braces") && !(b instanceof AST_Const)
             || output.option("ie8") && b instanceof AST_Do)
             return make_block(b, output);
         // The squeezer replaces "block"-s that contain only a single
@@ -1381,7 +1381,7 @@ function OutputStream(options) {
     });
 
     function force_statement(stat, output) {
-        if (output.option("braces")) {
+        if (output.option("braces") && !(stat instanceof AST_Const)) {
             make_block(stat, output);
         } else if (!stat || stat instanceof AST_EmptyStatement) {
             output.force_semicolon();