d3d12: fma cleanup
@jenatali: I recently realized that the situation had changed about our FMA usage, so perhaps it's about time to start using it when we can.
I kinda doubt this will affect much real-world code, but it might be worth giving a spin through the CL CTS, as that's what lead us to do things this way in the first place.
Dunno, is this worth it? It seems confusing to me to both have full lowering of the ffma opcodes, while handling it during codegen. That's kinda what triggered me to do this, but maybe we should just remove the codegen bits instead?