microsoft/compiler: correct dxil fma opcode

microsoft/compiler: correct dxil fma opcode

When I originally added the FFMA opcode here, I added the FMAD opcode
instead of the FMA opcode. The reason for this is that it works on
32-bit values as well, so that seemed like a better fit.

But that's not correct, as the FMA opcode isn't a fused operation, so
let's correct the opcode.

This isn't currently in use, because we currently lower away all ffma
opcodes on the NIR level, but that's about to change.

While we're at it, let's also update the opcode name to match the DXIL
documentation.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8349>