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

[Felipe de Azevedo Piovezan via llvm-commits]

felipepiovezan wrote:

> Interesting; I think falling back on SelectionDAG would produce a relatively normal result (emitting FAKE_USE),

To clarify, the entire _function_ would be skipped by GlobalISEL, not just the lowering of one instruction.

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