[ia32] Fix invalid DCHECK on cmpw with immediate.

The immediate passed to cmpw can be either a signed 16-bit or an
unsigned 16-bit integer, but the DCHECK was testing for signed 16-bit
values only.

R=mstarzinger@chromium.org
BUG=v8:6063

Review-Url: https://codereview.chromium.org/2735363002
Cr-Commit-Position: refs/heads/master@{#43671}

src/ia32/assembler-ia32.cc

@@ -841,7 +841,7 @@ void Assembler::cmpb(Register reg, const Operand& op) {
 
 
 void Assembler::cmpw(const Operand& op, Immediate imm16) {
-  DCHECK(imm16.is_int16());
+  DCHECK(imm16.is_int16() || imm16.is_uint16());
   EnsureSpace ensure_space(this);
   EMIT(0x66);
   EMIT(0x81);

test/mjsunit/regress/regress-6063.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.
+
+// Flags: --allow-natives-syntax
+
+var U16 = new Uint16Array(2);
+U16[0] = 0xffff;
+
+function foo(a, i) {
+  return U16[0] === 0xffff;
+}
+
+assertTrue(foo());
+assertTrue(foo());
+%OptimizeFunctionOnNextCall(foo);
+assertTrue(foo());