nak: Implement fmulz and ffmaz
Unfortunately, this involves touching core NIR a bit as we can't do fmulz
with denorms preserved. The DNZ bit implies FTZ so we have to tell NIR to only optimize of the client hasn't explicitly requested denorms. Fortunately, the one user of this is DXVK D3D9 and it never wants denorms anyway.
Closes: #10261 (closed)