[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[Wolfgang Pieb via llvm-commits]

wolfy1961 wrote:

> > So, depending on how/when these intrinsics are generated, this patch could affect a few different targets.
> 
> And it would introduce a situation we usually try to avoid: the presence of debug info could significantly change code generation.

Conceptually, though, extend-lifetimes is purely a codegen change. It could stand on its own, even though it would only be useful to improve debuggability. I think it's important to separate the 2 concepts. Debug info generation by itself does not trigger extend-lifetimes unless we specifically also request extend-lifetimes as well.

https://github.com/llvm/llvm-project/pull/86149