[llvm] [X86][APX] Implement push+push2+push pre-alignment strategy for PP2 (PR #205031)

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

> > Unwind v2 doesn't disassemble the epilog either, but it does require a "mirror" epilog as it assumes the prolog opcodes can be used to unwind in the middle of an epilog.
> 
> I think no exception would be triggered in the middle of an epilog in general.

Could be an AV from a corrupt stack, could be another thread aborting this one.

The unwind info can also be used during a stack walk to recover the current frames, for example during profiling or tracing (again, this may be from another thread).

Either way: the reason that unwind v2 requires the mirror epilog and unwind v3 capture the IP offset of **each** instruction in the epilog is to support this scenario - no matter what instruction we're at, we know how to unwind/walk.

https://github.com/llvm/llvm-project/pull/205031