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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

lgtm with nit. I think some more passes will need updating to deal with these (like InferAddressSpaces) but it's easiest to do that incrementally 

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