Added tests for saturation and some fixes for generic and avx target

tests/psubs_i8.ispc

@@ -0,0 +1,11 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform int8 a = -128, b = 127; // min and max signed int8
+    RET[programIndex] = psubs(a, b);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = -128;
+}