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

[David Blaikie via llvm-commits]

dwblaikie wrote:

A prologue begin/end solution only applies to top level locals in a function, right? For smaller scopes they won't extend that far and would need to be measured relative to the high/low/ranges bytes covered by the scope the variable is within (& of course that'll always neglect the issues of variables starting their lifetime part-way through their scope (`{ foo(); int x; foo(); }` won't be live for the first foo call, etc))

Though if llvm-dwarfdump statistics need improvement, I'd hope that could be done rather than building a statistic in a separate script, by the sounds of it?

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