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

[Stephen Tozer via llvm-commits]

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+# Parsing dwarfdump's output to determine whether the location list for the
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SLTozer wrote:

I agree in principle, the reason it's done here though is that the fake use can't guarantee liveness to the very end of the function, and so we can correctly lose "b" before a copy+return at the end of the function. The script adjusts for this by allowing a small numeric difference between the end of b's live range and the subprogram high PC; the way that I've updated the test to not be rotten means that _right now_ b's live range extends to the end of the function, so if that's good enough for now then I can rewrite this to use FileCheck.

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