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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> To clarify, the entire _function_ would be skipped by GlobalISEL, not just the lowering of one instruction.

Ah, I misunderstood - in which case forcing it to be dropped until a complete implementation is present would be preferable.

> Isn't the following statement contradicting the above?

`-Og` is also purely a codegen change, it doesn't control whether or not debug info is emitted; in both cases, we're telling the compiler to change codegen to be better for debugging.

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