[llvm] [LoopInterchange] Exit early in certain cases in legality check (NFC) (PR #139254)

[Michael Kruse via llvm-commits]

================
@@ -256,10 +260,19 @@ static bool isLegalToInterChangeLoops(CharMatrix &DepMatrix,
     // Create temporary DepVector check its lexicographical order
     // before and after swapping OuterLoop vs InnerLoop
     Cur = DepMatrix[Row];
-    if (!isLexicographicallyPositive(Cur))
+
+    // If the direction vector is lexicographically positive due to an element
+    // to the left of OuterLoopId, it is still positive after exchanging the two
+    // loops. In such a case we can skip the subsequent check.
----------------
Meinersbur wrote:

```suggestion
    // If the surrounding loops already ensure that the direction vector is lexicographically positive, nothing within the loop will be able to break the dependence.
    // In such a case we can skip the subsequent check.
```

https://github.com/llvm/llvm-project/pull/139254