[clang] [flang] [flang] add -floop-interchange and enable it with opt levels (PR #140182)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu commented:

Could you add a test that ensures that the loop-interchange pass is added to the pipeline. Perhaps something like [flang/test/Driver/slp-vectorize.f90](https://github.com/llvm/llvm-project/blob/04fde85057cb4da2e560da629df7a52702eac489/flang/test/Driver/slp-vectorize.f90#L9)

https://github.com/llvm/llvm-project/pull/140182