Fix desugaring of let bindings in for loops to handle continue properly

This requires putting the original loop's body inside an inner for loop (with
the same labels as the original loop) and re-binding the temp variables in its
"next" expression. A second flag is added to the desugared code to ensure the
loop body executes at most once per loop.

BUG=v8:3683
LOG=y

Review URL: https://codereview.chromium.org/720863002

Cr-Commit-Position: refs/heads/master@{#25363}

test/mjsunit/harmony/regress/regress-3683.js

+// Copyright 2014 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.
+//
+// Flags: --harmony-scoping
+
+"use strict";
+
+// Simplest case
+var count = 0;
+for (let x = 0; x < 10;) {
+  x++;
+  count++;
+  continue;
+}
+assertEquals(10, count);
+
+// Labeled
+count = 0;
+label: for (let x = 0; x < 10;) {
+  while (true) {
+    x++;
+    count++;
+    continue label;
+  }
+}
+assertEquals(10, count);
+
+// Simple and labeled
+count = 0;
+label: for (let x = 0; x < 10;) {
+  x++;
+  count++;
+  continue label;
+}
+assertEquals(10, count);
+
+// Shadowing loop variable in same scope as continue
+count = 0;
+for (let x = 0; x < 10;) {
+  x++;
+  count++;
+  {
+    let x = "hello";
+    continue;
+  }
+}
+assertEquals(10, count);
+
+// Nested let-bound for loops, inner continue
+count = 0;
+for (let x = 0; x < 10;) {
+  x++;
+  for (let y = 0; y < 2;) {
+    y++;
+    count++;
+    continue;
+  }
+}
+assertEquals(20, count);
+
+// Nested let-bound for loops, outer continue
+count = 0;
+for (let x = 0; x < 10;) {
+  x++;
+  for (let y = 0; y < 2;) {
+    y++;
+    count++;
+  }
+  continue;
+}
+assertEquals(20, count);
+
+// Nested let-bound for loops, labeled continue
+count = 0;
+outer: for (let x = 0; x < 10;) {
+  x++;
+  for (let y = 0; y < 2;) {
+    y++;
+    count++;
+    if (y == 2) continue outer;
+  }
+}
+assertEquals(20, count);