microsoft/compiler: do not lower away 64-bit ffma

In DXIL, the FMA instruction only supports 64-bit operations. However,
back when we implemented support for this, there were only a single
switch for lowering all ffma instructions, so we couldn't easily use it.

But now that there's separate flags to lower ffma on 16, 32 and 64 bit,
we can lower 16 and 32 bit ffmas, and leave 64 bit ffmas alone.

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

src/microsoft/compiler/nir_to_dxil.c

@@ -73,7 +73,6 @@ nir_options = {
    .lower_negate = true,
    .lower_ffma16 = true,
    .lower_ffma32 = true,
-   .lower_ffma64 = true,
    .lower_isign = true,
    .lower_fsign = true,
    .lower_iabs = true,