Prior to commit, many unit tests were implemented as `assert
tvm.ir.structural_equal(output, expected)`.  While this is correct, it
doesn't provide much information when the test fails.  The
`tvm.ir.assert_structural_equal` method performs the equivalent check,
but displays the exact location where a mismatch occurs.

This commit replaces all use of `assert tvm.ir.structural_equal` with
`tvm.ir.assert_structural_equal`.