glsl: Optimize clamp(x, 0, 1) as saturate(x)

glsl: Optimize clamp(x, 0, 1) as saturate(x)

v2: - Check that the base type is float (Ian Romanick)
v3: - Make sure comments reflect that we are doing a commutative operation
    - Add missing condition where the inner constant is 1.0 and outer constant is 0.0
    - Make indexing of operands easier to read (Matt Turner)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>