nir: Recognize max(min(a, 1.0), 0.0) as fsat(a).

nir: Recognize max(min(a, 1.0), 0.0) as fsat(a).

We already recognize min(max(a, 0.0), 1.0) as a saturate, but neglected
this variant (which is also handled by the GLSL IR pass).

shader-db results on Broadwell:
total instructions in shared programs: 7363046 -> 7362788 (-0.00%)
instructions in affected programs:     11928 -> 11670 (-2.16%)
helped:                                64
HURT:                                  0

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>