[turbofan] Fix more -Wsign-compare warnings.

test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc

Index: test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc
diff --git a/test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc b/test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc
index 5589ad26e89a4eb0af09c0156eaa4d60ca388436..3a2a26088a7a3e7e066490be1883b738b6c7c897 100644
--- a/test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc
+++ b/test/unittests/compiler/mips64/instruction-selector-mips64-unittest.cc
@@ -332,7 +332,7 @@ TEST_P(InstructionSelectorCmpTest, Parameter) {
 
   if (FLAG_debug_code &&
       type.representation() == MachineRepresentation::kWord32) {
-    ASSERT_EQ(6, s.size());
+    ASSERT_EQ(6U, s.size());
 
     EXPECT_EQ(cmp.mi.arch_opcode, s[0]->arch_opcode());
     EXPECT_EQ(2U, s[0]->InputCount());