Add reduce_equal() function to standard library.

tests/reduce-equal-13.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    int a = aFOO[programIndex&1];
+    RET[programIndex] = 1;
+    uniform bool re;
+    uniform int val;
+    if (programIndex & 1) {
+        re = reduce_equal(a, val);
+    }
+    RET[programIndex] = ((int)re << 8) + val;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 258;
+}