[SCEV] Swap guards estimation sequence. NFC

Summary:
Loop unroll spends a lot of time in SCEVs processing in case when a function
contains hundreds of simple 'for' loops with a quite complex arrays indexes like

  for (int i = 0; i < 8; ++i) {
    for (int j = 0; j < 32; ++j) {
      C[j*8+i] = B[j*32+i+128] + A[i*64+128];
    }
  }
  for (int i = 0; i < 8; ++i) {
    for (int j = 0; j < 8; ++j) {
      for (int k = 0; k < 32; ++k) {
        D[k*64+i*8+j] = D[k*64+i*8+j] + E[i+16] * C[k*8+j+256];
      }
    }
  }

The patch improves loop unroll speed since isLoopBackedgeGuardedByCond takes
much less time than isLoopEntryGuardedByCond in the edge case.

Reviewers: skatkov, sanjoy, mkazantsev

Reviewed By: sanjoy

Subscribers: fhahn, hiraditya, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72929

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