Add test that was supposed to go with 080241b7d1

tests/int64-constant.ispc

@@ -0,0 +1,12 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    int64 v = 0xffffffff;
+    RET[programIndex] = (v == 0xffffffffll) ? 1 : 0;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}