[llvm] [X86] Reimplement bundle alignment mode (PR #175830)

[Zachary Yedidia via llvm-commits]

zyedidia wrote:

Sounds good, I will give it some time and address remaining comments.

In an effort to reduce impact to the generic MC layer further, I've been experimenting with an approach that makes use of `AllowEnhancedRelaxation` and pushes more work to layout. It has the benefit of not needing to touch `MCObjectStreamer.cpp` at all (see https://github.com/lfi-project/llvm-project/tree/x86-bundling-simplification-2 for full details). This is a direction I can work towards further after this patch if that would be desirable.

https://github.com/llvm/llvm-project/pull/175830