[turbofan] Fix silly bug in loop variable analysis.

Review-Url: https://codereview.chromium.org/2222953003
Cr-Commit-Position: refs/heads/master@{#38448}

src/compiler/loop-variable-optimizer.cc

@@ -221,7 +221,7 @@ void LoopVariableOptimizer::VisitMerge(Node* node) {
   // Merge the limits of all incoming edges.
   VariableLimits* merged = limits_[node->InputAt(0)->id()]->Copy(zone());
   for (int i = 1; i < node->InputCount(); i++) {
-    merged->Merge(limits_[node->InputAt(0)->id()]);
+    merged->Merge(limits_[node->InputAt(i)->id()]);
   }
   limits_[node->id()] = merged;
 }

test/mjsunit/compiler/regress-loop-variable-if.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-loop-variable
+
+function f() {
+  for (var i = 0; i != 10; i++) {
+    if (i < 8) print("x");
+  }
+}
+
+f();