[llvm] [MC] Fix quadratic runtime within alignment boundary fragment relaxation (PR #176535)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay approved this pull request.

Sorry for taking a while to come back to this patch. I recall this patch as I work on a new fragment type to fix the recently-introduced `.prefalign` directive https://github.com/llvm/llvm-project/pull/184032#issuecomment-3982957269

Some nits

https://github.com/llvm/llvm-project/pull/176535