Do not reduce effect phis for loops.

This prevents eliminating effectful statements before the loop.

BUG=

Review URL: https://codereview.chromium.org/830923002

Cr-Commit-Position: refs/heads/master@{#25953}

src/compiler/control-reducer.cc

@@ -403,6 +403,14 @@ class ControlReducerImpl {
     if (n <= 1) return dead();            // No non-control inputs.
     if (n == 2) return node->InputAt(0);  // Only one non-control input.
 
+    // Never remove an effect phi from a (potentially non-terminating) loop.
+    // Otherwise, we might end up eliminating effect nodes, such as calls,
+    // before the loop.
+    if (node->opcode() == IrOpcode::kEffectPhi &&
+        NodeProperties::GetControlInput(node)->opcode() == IrOpcode::kLoop) {
+      return node;
+    }
+
     Node* replacement = NULL;
     Node::Inputs inputs = node->inputs();
     for (InputIter it = inputs.begin(); n > 1; --n, ++it) {

test/mjsunit/compiler/regress-ntl-effect.js

+// Copyright 2015 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 g() {
+  throw 0;
+}
+
+function f() {
+  g();
+  while (1) {}
+}
+
+assertThrows(function () { f(); });