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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> I only skimmed the diff, but noticed there is no global isel lowering here. Were there any challenges in doing that or was it not considered?

I wasn't the original patch writer, but I think it would be the latter. I'm not familiar with the internals of GlobalISel, so I'm not sure how confident I'd be implementing support for fake use there myself - though FWIW, since it's a no-op it's mostly just filling in the boilerplate, with a few more complex cases (such as moving fake uses before tail calls).

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