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

tests/exclusive-scan-add-6.ispc

@@ -0,0 +1,12 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = exclusive_scan_add((float)programIndex); 
+}
+
+export void result(uniform float RET[]) {
+    uniform int result[] = { 0, 0, 1, 3, 6, 10, 15, 21, 28,
+                             36, 45, 55, 66, 78, 91, 105, 120 };
+    RET[programIndex] = result[programIndex]; 
+}