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

[Paul T Robinson via llvm-commits]

pogo59 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?

Our target of interest is X86, so I believe it was not considered. @wolfy1961 can correct me if I'm mis-remembering.

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