nir: allow sat on all float destination types

nir: allow sat on all float destination types

With the introduction of fp64 and fp16 to nir, there are now a bunch of
float types running around. A F1 2015 shader ends up with an i2f.sat
operation, which has a nir_type_float32 destination. Allow sat on all
the float destination types.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>