Add test to confirm that we forbid allocate statement referencing undefined variable (#1899)

tests/python/unittest/test_pass_split_host_device.py

+from nose.tools import raises
+import tvm
+
+@raises(Exception)
+def test_loop_dependent_allocate():
+    N = tvm.var("N")
+    A = tvm.placeholder((2*N,), "float32", "A")
+    C = tvm.compute((N, ), lambda i: A[2*i] + A[i+1], name='C')
+    s = tvm.create_schedule(C.op)
+    AA = s.cache_read(A, "local", [C])
+    s[AA].compute_at(s[C], s[C].op.axis[0])
+    # this line should fail due to IRUseDefAnalysis sees an allocate statement
+    # referencing undefined variable
+    tvm.lower(s, [A,C])
+
+if __name__ == "__main__":
+    test_loop_dependent_allocate()