[x86] Fix register constraints for multiply-high.

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

Index: test/unittests/compiler/x64/instruction-selector-x64-unittest.cc
diff --git a/test/unittests/compiler/x64/instruction-selector-x64-unittest.cc b/test/unittests/compiler/x64/instruction-selector-x64-unittest.cc
index 8470818464b6e179b408c4b17a39c4f332d77051..dddb70c58d44d27adc48649b44aba21faad2c34e 100644
--- a/test/unittests/compiler/x64/instruction-selector-x64-unittest.cc
+++ b/test/unittests/compiler/x64/instruction-selector-x64-unittest.cc
@@ -510,9 +510,12 @@ TEST_F(InstructionSelectorTest, Int32MulHigh) {
   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