[turbofan] Use CompareNilIC for abstract equality with null/undefined.

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index 7134bb221e406894222e36ff7810099492140cec..a776fa18acdc1cf1ff5ca27eb23f0e69da3d31cb 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -4881,8 +4881,8 @@ void FullCodeGenerator::EmitLiteralCompareNil(CompareOperation* expr,
   } else {
     Handle<Code> ic = CompareNilICStub::GetUninitialized(isolate(), nil);
     CallIC(ic, expr->CompareOperationFeedbackId());
-    __ testp(rax, rax);
-    Split(not_zero, if_true, if_false, fall_through);
+    __ CompareRoot(rax, Heap::kTrueValueRootIndex);
+    Split(equal, if_true, if_false, fall_through);
   }
   context()->Plug(if_true, if_false);
 }