nir_to_dxil has store_def and store_ssa_def and they're subtly different
This is an artifact of all the NIR clean-ups we've been doing. @alyssa and I have been moving code around like crazy and sometimes the obvious code motion leaves things in a weird state. It's all correct as far as I can tell, things just have funny names now. It looks like the two can and should be rolled into one function but I don't have the DXIL knowledge to know if that's a good idea or even correct.