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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> 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

Thanks very much for the amount of work put into this review - I'll rebase now and highlight if there are any notable conflicts/changes before merging.

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