[turbofan] Teach escape analysis about StringCharAt

R=bmeurer@chromium.org
BUG=chromium:677757

Review-Url: https://codereview.chromium.org/2606383005
Cr-Commit-Position: refs/heads/master@{#42066}

src/compiler/escape-analysis.cc

@@ -818,6 +818,7 @@ bool EscapeStatusAnalysis::CheckUsesForEscape(Node* uses, Node* rep,
       case IrOpcode::kPlainPrimitiveToNumber:
       case IrOpcode::kPlainPrimitiveToWord32:
       case IrOpcode::kPlainPrimitiveToFloat64:
+      case IrOpcode::kStringCharAt:
       case IrOpcode::kStringCharCodeAt:
       case IrOpcode::kObjectIsCallable:
       case IrOpcode::kObjectIsNumber:

test/mjsunit/regress/regress-crbug-677757.js

+// Copyright 2017 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.
+
+for (var i = 0; i < 50000; i++) {
+ ("Foo"[0] + "barbarbarbarbarbar")[0];
+}