[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