[llvm] [LoopInterchange] Enable it by default (WIP) (PR #124911)

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

Re: perf number, @Meinersbur : I indeed see no regressions and 3 improvements of ~70% that are real, I think. There are a couple of minor improvements, but that could be real or noise, but didn't look further into it. I did 3 runs with interchange disabled ("before") and 3 runs with interchange enabled ("after"), and lower is better:

```
Program                                                 exec_time
                                                        before1   before2 before3 after1 after2 after3 diff
Polybench/linear-algebra/solvers/cholesky/cholesky.test  11.48     11.34   11.20    6.60   6.58   6.63 74.5%
    Polybench/linear-algebra/solvers/ludcmp/ludcmp.test  23.56     23.57   23.55   14.16  14.13  14.08 67.5%
            Polybench/linear-algebra/solvers/lu/lu.test  23.60     23.48   23.69   14.25  14.17  14.21 67.2%
```

https://github.com/llvm/llvm-project/pull/124911