Some fixes in function names and more tests was added.

tests/psubs_i16-2.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 int16 a = 32767; // max signed int16
+    RET[programIndex] = saturating_sub(a, -b);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = (uniform int16) 32767;
+}