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

tests/exclusive-scan-and-1.ispc

@@ -0,0 +1,22 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = -1;
+    int32 a = (programIndex & 1) ? 0xff : 0;
+    if (programIndex & 1) {
+        RET[programIndex] = exclusive_scan_and(a);
+    }
+}
+
+
+export void result(uniform float RET[]) {
+    if (programIndex & 1) {
+        if (programIndex == 1)
+            RET[programIndex] = -1; // 0xffffffff, made float
+        else
+            RET[programIndex] = 0xff;
+    }
+    else
+        RET[programIndex] = -1;
+}