[LoopInterchange] Relax the legality check to accept more patterns (#139690) When proving the legality of exchanging two loops, it doesn't need to check the elements of the direction vectors associated with the loops outside of the two target loops. Before this patch, the legality check looked at all elements of a direction vector to calculate the lexicographically order of the vector, which may reject some legal exchanges. For example, if a direction vector is `[* < =]`, it is safe to swap the last two loops because the corresponding subsequence of the vector (`[< =]`) is lexicographically positive for both before and after the exchange. However, the its order is unknown if we don't drop the prefix since the first element is `*`. This patch improves the logic of legality check to ignore such unrelated prefixes of direction vectors.
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