Add some additional tests

tests/vector-varying-scatter.ispc

@@ -0,0 +1,15 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float aa = aFOO[programIndex]; 
+    float<4> a = { -1, -2, -3, -4 };
+    if (programIndex < 4)
+        a[3-programIndex] = aa;
+    RET[programIndex] = a[2];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = -3;
+    RET[1] = 2;
+}