glsl: Optimize clamp(x, b, 1.0), where b > 0.0 as max(saturate(x),b)

glsl: Optimize clamp(x, b, 1.0), where b > 0.0 as max(saturate(x),b)

v2: - Output max(saturate(x),b) instead of saturate(max(x,b))
    - Make sure we do component-wise comparison for vectors (Ian Romanick)
v3: - Add missing condition where the outer constant value is > 0.0 and
      inner constant is 1.0.
    - Fix comments to show that the optimization is a commutative operation
      (Matt Turner)

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