Add more tests of basic gather functionality.

tests/gather-int64-2.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+int64 zero = 0;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform int64 a[programCount];
+    for (uniform int i = 0; i < programCount; ++i)
+        a[i] = aFOO[i];
+    
+    int g = a[programIndex+zero]; 
+    RET[programIndex] = g; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1 + programIndex;
+}