[llvm] [Passes] Move LoopInterchange into optimization pipeline (PR #145503)

[Ryotaro Kasuga via llvm-commits]

================
@@ -1547,6 +1544,10 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   //        this may need to be revisited once we run GVN before loop deletion
   //        in the simplification pipeline.
   LPM.addPass(LoopDeletionPass());
+
+  if (PTO.LoopInterchange)
+    LPM.addPass(LoopInterchangePass());
----------------
kasuga-fj wrote:

Yes, collecting diverse data would be ideal, but since the current LoopInterchange is rarely triggered in practice, I haven’t been able to gather enough data to have a meaningful discussion about the optimal placement...

[TSVC s235](https://github.com/llvm/llvm-test-suite/blob/b9f9a11a1441741a3bc6e7221854418f26e9f465/MultiSource/Benchmarks/TSVC/tsc.inc#L2218-L2244) might be an interesting example. If LoopDistribute were extended to support non-innermost loops and it ran before LoopInterchange, the following could become possible:

Original:
```c
for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int i = 0; i < LEN2; i++) {
    a[i] += b[i] * c[i];
    for (int j = 1; j < LEN2; j++) {
      aa[j][i] = aa[j-1][i] + bb[j][i] * a[i];
    }
  }
```

Distributed:
```c
for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int i = 0; i < LEN2; i++)
    a[i] += b[i] * c[i];

for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int i = 0; i < LEN2; i++)
    for (int j = 1; j < LEN2; j++)
      aa[j][i] = aa[j-1][i] + bb[j][i] * a[i];
```

Interchanged:
```c
for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int i = 0; i < LEN2; i++)
    a[i] += b[i] * c[i];

// The i-loop and j-loop are interchanged.
for (int nl = 0; nl < 200*(ntimes/LEN2); nl++)
  for (int j = 1; j < LEN2; j++)
    for (int i = 0; i < LEN2; i++)
      aa[j][i] = aa[j-1][i] + bb[j][i] * a[i];
```

IIRC, when I ran the last code in the past, it was about 8x faster than the original. But achieving this would require a lot of work on LoopDistribute... 

https://github.com/llvm/llvm-project/pull/145503