[PowerPC][NFC] Update test to prevent DCE from causing failures

The test case provided in PR45709 can be simplified by DCE to an
empty function. To prevent this from happening if DCE is run prior
to ISEL in the back end, just add optnone to the function. The
behaviour it is testing for is in the SDAG legalization and is
not sensitive to optnone so the test case still achieves its desired
objective.