add simplify test (#12)

HalideIR @ 32781037

-Subproject commit 98e8df564f8543b337ec0528dbcb06a30f91e694
+Subproject commit 3278103721cfabf7435f1e9ba1fd75a7c38f13c9

tests/cpp/ir_simplify_test.cc

+#include <dmlc/logging.h>
+#include <gtest/gtest.h>
+#include <tvm/tvm.h>
+#include <pass/Simplify.h>
+
+TEST(IRSIMPLIFY, Basic) {
+  using namespace Halide::Internal;
+  simplify_test();
+}
+
+int main(int argc, char ** argv) {
+  testing::InitGoogleTest(&argc, argv);
+  testing::FLAGS_gtest_death_test_style = "threadsafe";
+  return RUN_ALL_TESTS();
+}