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

[Sjoerd Meijer via cfe-commits]

sjoerdmeijer wrote:

For more context, this is part of our loop-interchange enablement story, see our RFC here: https://discourse.llvm.org/t/enabling-loop-interchange/82589.

We have fixed all the compile-time issues and loop-interchange issues that we are aware of, and would like to enable this in the C/C++ flow, see here: https://github.com/llvm/llvm-project/pull/124911.

As part of this work, we also promised to fix DependenceAnalysis. The last DA correctness corner-case that is being worked on is: https://github.com/llvm/llvm-project/pull/123436. This is a corner-case for C/C++ related to type punning, different offset sizes that won't be a problem in Fortran. Therefore, we think that enabling interchange and dependence analysis for Fortran makes sense.

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