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

[Paul T Robinson via llvm-commits]

pogo59 wrote:

> isDebugInstr might also be worth considering, but I'm guessing it's not really a debug instruction?

FAKE_USE influences code generation so as a matter of principle it should not be thought of as a debug instruction. (Those are going away anyhow, but there's a dividing line there.)

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