[x86] Fix register constraints for multiply-high.

test/unittests/compiler/x64/instruction-selector-x64-unittest.cc

 // Copyright 2014 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.
 
 #include "test/unittests/compiler/instruction-selector-unittest.h"
 
 #include "src/compiler/node-matchers.h"
 
 namespace v8 {
 namespace internal {
@@ skipping 492 matching lines @@
   StreamBuilder m(this, kMachInt32, kMachInt32, kMachInt32);
   Node* const p0 = m.Parameter(0);
   Node* const p1 = m.Parameter(1);
   Node* const n = m.Int32MulHigh(p0, p1);
   m.Return(n);
   Stream s = m.Build();
   ASSERT_EQ(1U, s.size());
   EXPECT_EQ(kX64ImulHigh32, s[0]->arch_opcode());
   ASSERT_EQ(2U, s[0]->InputCount());
   EXPECT_EQ(s.ToVreg(p0), s.ToVreg(s[0]->InputAt(0)));
+  EXPECT_TRUE(s.IsFixed(s[0]->InputAt(0), rax));
   EXPECT_EQ(s.ToVreg(p1), s.ToVreg(s[0]->InputAt(1)));
+  EXPECT_TRUE(!s.IsUsedAtStart(s[0]->InputAt(1)));
   ASSERT_EQ(1U, s[0]->OutputCount());
   EXPECT_EQ(s.ToVreg(n), s.ToVreg(s[0]->Output()));
+  EXPECT_TRUE(s.IsFixed(s[0]->OutputAt(0), rdx));
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8