[crankshaft] Properly handle OOB string accesses.

BUG=chromium:665793

Review-Url: https://codereview.chromium.org/2589823003
Cr-Commit-Position: refs/heads/master@{#41842}

src/crankshaft/hydrogen.cc

@@ -7508,6 +7508,12 @@ void HOptimizedGraphBuilder::BuildLoad(Property* expr,
     HValue* string = Pop();
     HInstruction* char_code = BuildStringCharCodeAt(string, index);
     AddInstruction(char_code);
+    if (char_code->IsConstant()) {
+      HConstant* c_code = HConstant::cast(char_code);
+      if (c_code->HasNumberValue() && std::isnan(c_code->DoubleValue())) {
+        Add<HDeoptimize>(DeoptimizeReason::kOutOfBounds, Deoptimizer::EAGER);
+      }
+    }
     instr = NewUncasted<HStringCharFromCode>(char_code);
 
   } else if (expr->key()->IsPropertyName()) {

test/mjsunit/regress/regress-crbug-665793.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: --allow-natives-syntax
+
+function foo() {
+  return 'x'[1];
+}
+assertEquals(undefined, foo());
+%OptimizeFunctionOnNextCall(foo);
+assertEquals(undefined, foo());