LoopVectorizer depends on ifthenelse

[yongfeng-nv]

My PR (#4885) is blocked by a test failure (https://ci.tvm.ai/blue/organizations/jenkins/tvm/detail/PR-4885/1/pipeline).
The bug can be reproduced without my change. Pick this commit (yongfeng-nv@42cef28) and run tests/python/relay/test_op_level4.py to reproduce the failure.

The original test passes the following IR to LoopVectorizer:

produce strided_set {
  parallel (i0.i1.fused, 0, 12) {
    vectorized (i2.inner, 0, 16) {
      if ((i2.inner < 3)) {
        if ((i2.inner < 3)) {
            strided_set[((i0.i1.fused*16) + i2.inner)] = tvm_if_then_else(...
        }
      }
    }
  }
}

The modified test passes this:

produce strided_set {
  parallel (i0.i1.fused, 0, 12) {
    vectorized (i2.inner, 0, 16) {
      strided_set[((i0.i1.fused*16) + i2.inner)] = tvm_if_then_else(...
    }
  }
}

The "if" statements make some difference, but I don't know the exact impact. My PR removes the "if" statements from the original test and exposes this bug.

[yongfeng-nv]

#4916 fixed this issue. Close it now.