[clang] [flang] [Clang][Flang] Enable loop interchange by default at -O3 (PR #216920)

[Nikita Popov via cfe-commits]

nikic wrote:

Ooops.

> Default the -floop-interchange flag on at -O3 for both frontends so they match the pipeline default.

It looks like PassBuilderPipelines does not actually condition LoopInterchange on O3. Should we match that and enable it for all opt levels? The pass is quite cheap now, so I don't mind.

https://github.com/llvm/llvm-project/pull/216920