Add more tests of basic gather functionality.

tests/gather-int8-8.ispc

@@ -0,0 +1,23 @@
+
+export uniform int width() { return programCount; }
+
+int64 zero = 0;
+void *gptr;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform int8 a[programCount];
+    for (uniform int i = 0; i < programCount; ++i)
+        a[i] = aFOO[i];
+    
+    int g = 0;
+    int8 *ptr = (aFOO[0] == 1234) ? (int8 * varying)gptr : (a + programIndex);
+    if (programIndex < 2)
+        g = *ptr;
+    RET[programIndex] = g; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+    RET[0] = 1;
+    RET[1] = 2;
+}