[turbofan] simplified lowering: process DeadValue input

Without processing the input, a phi node can be flagged as unused and
replaced with {Dead}, although it is used by a {DeadValue} node.

Bug: chromium:808472
Change-Id: I7446883535b34770e31e4e26e1c242eb05673a91
Reviewed-on: https://chromium-review.googlesource.com/919362
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51395}

src/compiler/simplified-lowering.cc

@@ -3060,6 +3060,7 @@ class RepresentationSelector {
         // Assume the output is tagged.
         return SetOutput(node, MachineRepresentation::kTagged);
       case IrOpcode::kDeadValue:
+        ProcessInput(node, 0, UseInfo::Any());
         return SetOutput(node, MachineRepresentation::kNone);
       default:
         FATAL(

test/mjsunit/compiler/regress-808472.js

+// Copyright 2018 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.
+
+function opt() {
+    let opt, arr = [...[...[...[...new Uint8Array(0x10000)]]]];
+    while (arr--)  {
+        opt = ((typeof opt) === 'undefined') ? /a/ : arr;
+    }
+}
+opt();
+opt();

test/mjsunit/mjsunit.status

@@ -104,6 +104,7 @@
   'generated-transition-stub': [PASS, ['mode == debug', SKIP]],
   'migrations': [SKIP],
   'array-functions-prototype-misc': [PASS, SLOW, ['mode == debug', SKIP]],
+  'compiler/regress-808472': [PASS, ['mode == debug', SKIP]],
 
   ##############################################################################
   # This test sets the umask on a per-process basis and hence cannot be