Add reduce_equal() function to standard library.

tests/reduce-equal.ispc

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