vm/bench: extend refcounter benchmarks

Adding an array multiple times leads to the fast update via `IncRC`.
This hides the allocation that is there on the first addition. In this
commit add another benchmark which measures Add/Remove together, to
ensure that `switch` in `refCounter.Add` is entered. Benchmark results
are meaningful, because `Add`/`Remove` have almost identical implementation.

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/vm/ref_counter_test.go

@@ -55,3 +55,14 @@ func BenchmarkRefCounter_Add(b *testing.B) {
 		rc.Add(a)
 	}
 }
+
+func BenchmarkRefCounter_AddRemove(b *testing.B) {
+	a := stackitem.NewArray([]stackitem.Item{})
+	rc := newRefCounter()
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		rc.Add(a)
+		rc.Remove(a)
+	}
+}