nir: Combine lower_fmod16/32 back into a single lower_fmod.

nir: Combine lower_fmod16/32 back into a single lower_fmod.

We originally had a single lower_fmod option.  In commit 2ab2d2e5, Sam
split 32 and 64-bit lowering into separate flags, with the rationale
that some drivers might want different options there.  This left 16-bit
unhandled, so Iago added a lower_fmod16 option in commit ca31df6f.

Now that lower_fmod64 is gone (in favor of nir_lower_doubles and
nir_lower_dmod), we re-combine lower_fmod16 and lower_fmod32 into a
single lower_fmod flag again.  I'm not aware of any hardware which
need lowering for one bitsize and not the other.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>