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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

Just fixed a minor error where X86 and AArch64 tests were bundled in a single file - separated them out into `X86/fake-use.ll` and `AArch64/fake-use-global-isel.ll`, and added a small additional catch in the check script. Since this is a minor test change, I'll proceed with the merge and take post-commit review on those tests.

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