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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> It's definitely possible:

Ah that's true, if it's going to be live at a single location for every instruction in the function then it'll emit with a fixed location. That shouldn't happen in this test since we have the big register kill operation, but no point in making assumptions when we can add coverage instead - will update.

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