adding optional else to executable semantics (#295)

* adding comments and some newlines

* added optonal else

executable_semantics/testdata/if3.6c

@@ -0,0 +1,13 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn main() -> Int {
+  if (0 == 0)
+    if (0 == 1)
+      return 1;
+    else
+      return 0;
+
+  return 1;
+}