[llvm] Recommit "[LoopFlatten] Enable it by default" (PR #90473)

[Nikita Popov via llvm-commits]

https://github.com/nikic requested changes to this pull request.

It looks like this pass has become unacceptably slow in the meantime: https://llvm-compile-time-tracker.com/compare.php?from=e1622e189e8c0ef457bfac528f90a7a930d9aad2&to=e7f00b505e9e8a7c6781e3ceb8e35bb4c8d94908&stat=instructions:u

For comparison, these are numbers for this pass from ~2.5 years ago: https://llvm-compile-time-tracker.com/compare.php?from=6f45fe9851c673883b3a258351ee4997aa2c028c&to=439d1c9613828db24ad03eb415baad2e76b8913c&stat=instructions:u

https://github.com/llvm/llvm-project/pull/90473