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

tests/exclusive-scan-and-2.ispc

@@ -0,0 +1,21 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = -1;
+    int32 a = ~(1 << programIndex);
+    if ((programIndex & 1) == 0) {
+        RET[programIndex] = exclusive_scan_and(a);
+    }
+}
+
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = -1;
+    if ((programIndex & 1) == 0 && programIndex > 0) {
+        int val = 0xffffffff;
+        for (int i = 0; i < programIndex-1; i += 2)
+            val &= ~(1<<i);
+        RET[programIndex] = val;
+    }
+}