[llvm] [MC][ELF] Emit instructions directly into fragment (PR #94950)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Nice! With `Code` removal in #95188 and the latest update, emitting instructions directly into `DF->getContents()` can be enabled with very little code.

I think there will be some memory consumption bump for aligned bundling, but it is unlikely anyone cares about that particularly now.

https://github.com/llvm/llvm-project/pull/94950