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

tests/exclusive-scan-or-1.ispc

@@ -0,0 +1,13 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = -1;
+    int32 a = (1 << programIndex);
+    RET[programIndex] = exclusive_scan_or(a);
+}
+
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = (1 << programIndex) - 1;
+}