glsl: Optimize clamp(x, 0.0, b), where b < 1.0 as min(saturate(x),b)

glsl: Optimize clamp(x, 0.0, b), where b < 1.0 as min(saturate(x),b)

v2: - Output min(saturate(x),b) instead of saturate(min(x,b)) suggested by Ilia Mirkin
    - Make sure we do component-wise comparison for vectors (Ian Romanick)
v3: - Add missing condition where the outer constant value is zero and
      inner constant is < 1
    - Fix comments to reflect we are doing a commutative operation (Matt Turner)

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