[turbofan] Properly kill Terminate nodes when removing loops.

BUG=chromium:491578
LOG=n
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1161583002

Cr-Commit-Position: refs/heads/master@{#28621}

src/compiler/control-reducer.cc

@@ -439,10 +439,16 @@ class ControlReducerImpl final : public AdvancedReducer {
 
     if (live == 0) return dead();  // no remaining inputs.
 
-    // Gather phis and effect phis to be edited.
+    // Gather terminates, phis and effect phis to be edited.
     NodeVector phis(zone_);
+    Node* terminate = nullptr;
     for (Node* const use : node->uses()) {
-      if (NodeProperties::IsPhi(use)) phis.push_back(use);
+      if (NodeProperties::IsPhi(use)) {
+        phis.push_back(use);
+      } else if (use->opcode() == IrOpcode::kTerminate) {
+        DCHECK_NULL(terminate);
+        terminate = use;
+      }
     }
 
     if (live == 1) {
@@ -450,6 +456,8 @@ class ControlReducerImpl final : public AdvancedReducer {
       for (Node* const phi : phis) {
         Replace(phi, phi->InputAt(live_index));
       }
+      // The terminate is not needed anymore.
+      if (terminate) Replace(terminate, dead());
       // The merge itself is redundant.
       return node->InputAt(live_index);
     }

test/mjsunit/compiler/regress-491578.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 foo(x) {
+  if (x === undefined) return;
+  while (true) {
+    while (1 || 2) { }
+    f();
+  }
+}
+%OptimizeFunctionOnNextCall(foo);
+foo();