[llvm] MC: Store MCRelaxableFragment MCInst out-of-line (PR #147229)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke commented:

Tentative LGTM. MCFragment is now always trivially destructible, which is good. MCInst re-constructions seems a bit hacky, but ok. Flags might be a problem. This introduces 1--2 additional operand copies for relaxed instructions, but most instructions go into data fragments, so the impact is negligible.

https://github.com/llvm/llvm-project/pull/147229