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

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

> 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.

Ah ok if it can't occur in the test then no need. We can extend it as and when necessary (that's better than having additional complexity that isn't used in the script).

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