Some saturation tests fixes

tests/psubus_i16.ispc

@@ -2,7 +2,7 @@
 export uniform int width() { return programCount; }
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
-    uniform int8 a = 0, b = 32767; // min and max unsigned int16
+    uniform unsigned int8 a = 0, b = 32767; // min and max unsigned int16
     RET[programIndex] = psubus(a, b);
 }