intel/fs: Don't assert on b2f with a saturate modifier

intel/fs: Don't assert on b2f with a saturate modifier

This ran afoul of Iris's use of nir_lower_clamp_color_outputs which
applies fsat() before writes to vertex shader color outpus.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Fixes: 7725d609387 ("intel/fs: Emit better code for b2f(inot(a)) and b2i(inot(a))")