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

[Tobias Stadler via llvm-commits]

tobias-stadler wrote:

Is there a good reason for not putting isFakeUse() into MachineInstr::isPosition() instead of sprinkling it over the codebase?
Putting into isPosition would make it a scheduling boundary, ignored by CSE, unsafe to move and therefore ignored by DCE.

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