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

tests/exclusive-scan-add-10.ispc

@@ -0,0 +1,20 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = -1;
+    unsigned int64 a = aFOO[programIndex];
+    if (programIndex & 1) {
+        RET[programIndex] = exclusive_scan_add(a);
+    }
+}
+
+
+export void result(uniform float RET[]) {
+    uniform int result[16] = { 0, 0, 0, 2, 0, 6, 0, 12, 
+                               0, 20, 0, 30, 0, 42, 0, 56 };
+    if (programIndex & 1)
+        RET[programIndex] = result[programIndex]; 
+    else
+        RET[programIndex] = -1;
+}