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 <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Signed-off-by: Abdiel Janulgue <b6177409facd0a1ebaf54f9c29b28b2642b3c468@linux.intel.com>

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