Some fixes in function names and more tests was added.

tests/padds_i8-2.ispc

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