Allow empty first parts of ConsStrings

TurboFan lowering (see [0]) of ConsString creation cannot ensure that
the first part of the cons string is non-empty without introducing a phi
and negatively impacting performance.

This modifies ConsStringIterator to allow empty first parts of
ConsStrings.

BUG=v8:5440

Review-Url: https://codereview.chromium.org/2377983002
Cr-Commit-Position: refs/heads/master@{#39817}

src/objects.cc

@@ -11092,7 +11092,7 @@ String* ConsStringIterator::NextLeaf(bool* blew_stack) {
       if ((type & kStringRepresentationMask) != kConsStringTag) {
         AdjustMaximumDepth();
         int length = string->length();
-        DCHECK(length != 0);
+        if (length == 0) break;  // Skip empty left-hand sides of ConsStrings.
         consumed_ += length;
         return string;
       }

test/mjsunit/regress/regress-5440.js

+// Copyright 2016 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: --turbo --always-opt
+
+// The rightmost cons string is created first, resulting in an empty left part.
+eval(" " + ("" + "try {;} catch (_) {}"));