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

[Nikita Popov via cfe-commits]

nikic wrote:

To be honest, I am very surprised that this PR was submitted, let alone landed. We have **explicitly not approved** enabling LoopInterchange by default in LLVM due to outstanding issues. So instead you go ahead and enable it directly in Flang instead, without even notifying any of the people who were involved in the original discussions. That's not how things should work.

Please revert this patch ASAP and request an LLVM 21 backport.

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