Add reduce_equal() function to standard library.

tests/reduce-equal-1.ispc

@@ -0,0 +1,11 @@
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    int a = b;
+    RET[programIndex] = reduce_equal(a) ? 1 : 0; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}