[LoopInterchange] Fix legality for triangular loops

This is a bug fix in legality check.

When we encounter triangular loops such as the following form:
    for (int i = 0; i < m; i++)
      for (int j = 0; j < i; j++), or

    for (int i = 0; i < m; i++)
      for (int j = 0; j*i < n; j++),

we should not perform interchange since the number of executions
of the loop body will be different before and after interchange,
resulting in incorrect results.

Reviewed By: bmahjour

Differential Revision: https://reviews.llvm.org/D101305