Add support for scan operations across program instances (add, and, or).

tests/exclusive-scan-add-8.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = -1;
+    int64 a = aFOO[programIndex];
+    if (a <= 2)
+        RET[programIndex] = exclusive_scan_add(a);
+}
+
+export void result(uniform float RET[]) {
+    uniform int result[] = { 0, 1, 3, 0, 0, 0, 0, 0,
+                             0, 0, 0, 0, 0, 0, 0, 0 };
+    RET[programIndex] = -1;
+    if (programIndex <= 1)
+        RET[programIndex] = result[programIndex]; 
+}