Ensure hole checks take place in switch statement scopes

test/mjsunit/regress/regress-3926.js

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// See: http://code.google.com/p/v8/issues/detail?id=3926
+
+// Switch statements should disable hole check elimination
+
+'use strict';
+
+function f(x) {
+  var z;
+  switch (x) {
+    case 1:
+      let y = 1;
+    case 2:
+      y = 2;
+    case 3:
+      z = y;
+  }
+  return z;
+}
+assertEquals(2, f(1));
+assertThrows(function() {f(2)}, ReferenceError);
+assertThrows(function() {f(3)}, ReferenceError);
+
+assertThrows(function() {
+  switch (1) {
+    case 0:
+      let x = 2;
+    case 1:
+    { // this block, plus the let below, adds another linear lexical scope
+      let y = 3;
+      x;
+    }
+  }
+}, ReferenceError);