[llvm] [MC] Maintain MCRelaxAll after reset() (PR #94945)

[Fangrui Song via llvm-commits]

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

LGTM. Eventually we should probably add a testing-only option to call `reset` early so that these stuff is more testable.

https://github.com/llvm/llvm-project/pull/94945