glsl: Add a pass to lower ir_unop_saturate to clamp(x, 0, 1)

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

src/glsl/ir_optimization.h

@@ -40,6 +40,7 @@
 #define LDEXP_TO_ARITH     0x100
 #define CARRY_TO_ARITH     0x200
 #define BORROW_TO_ARITH    0x400
+#define SAT_TO_CLAMP       0x800
 
 /**
  * \see class lower_packing_builtins_visitor